I got the extension pretty hot. The 620 got soft but never flowed. However, once it got a little too hot it hardened back up considerably. Either way, it isn't going to move once it sets up and I would think heat cycling it would make it harder. In this case, the barrel had been mounted but never fired.
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Is bedding the AR barrel a myth? What's the best way?
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I got the extension pretty hot. The 620 got soft but never flowed. However, once it got a little too hot it hardened back up considerably. Either way, it isn't going to move once it sets up and I would think heat cycling it would make it harder. In this case, the barrel had been mounted but never fired.
Nice! That eases some doubts that I had. I figured it was fairly durable if people used for certain jobs like cylinder replacement in engines.
Not too hard was it. If I remember the article I referenced from Joe Carlos, he suggested 42ish ft lbs. I have no idea where the number came from (probably lots of testing I hope) but I torque to 45 ft lbs on a shimmed upper because I just can't do 42....
I've taken blue Loctite off of a few extensions and it doesn't like to come off. Cussing it seemed to help.
52ft lb is my lower limit for small frames. I only go that low for an aluminum nut (I have 1 on a 10.5" upper) or if I'm worried about breaking teeth off a steel nut.
60-65 is my preference with well lubricated threads. Steel or titanium nuts preferred.
I've always trued receiver faces & shimmed non thermal uppers, but recently did a test on this to satisfy my own curiosity.
https://www.snipershide.com/shooting/threads/facing-an-ar-receiver-does-it-matter.7134714/
The Joe Carlos videos are great & I linked one in the above thread. Keep in mind though he was playing within a specific set of rules that include factory ammo. We have free range to choose what we use. The concepts are solid, but there affect will vary.
The main take away for me, with that test was that 62ft lbs appears to pin the shoulder pretty well. I think a lot of poeple don't torque AR nuts high enough which would exacerbate a loose receiver. It takes considerable torque just to overcome thread friction on large threads. The test wasn't exactly scientific & by chance that upper had the squarest face of any I've ever trued. Therefor the test didn't really provide anything usefull as far as squaring the receiver. The extension fit was very loose (took a full wrap .001 shim with 620). Every VLTOR upper I've put together has been loose. FWIW I also use 620 for the gas block to seal it.
It's now shimmed. I never updated the thread beyond factory loads, but the gun hammers with my 77smk load & doesn't start stringing shots when it heats up.
As for the shifting poi, I remember a big thread on here about that Drake prior to any Springfield involvement. The consensus was, it's ugly, but had merit. IIRC they were using cut rifled Satern Barrels.
Personally I like true monolithic uppers. I have a 6ARC VLTOR fusion with a 15" rail set up for matches & have not seen any poi shift from bag to bipod. Just re-barreled it. Talk about a PITA to shim. It balances well on a bag. Need to get some rounds on it. Maybe I'll do a poi test @200y with 20 rounds each.
Found this video a couple years ago on how JP thermal fits its uppers to its barrels. It worked on a 308 upper. I used stainless shim stock on an udersized barrel extension and was somehow able to hold it all together long enough to stuff the extension into the hot upper.
So, just to clarify...
Even with a thermal fit, folks are still shimming/bedding their barrels and seeing improvements in accuracy? Or is this just for non-thermal fit barrels?
I think I want to try this. It looks pretty straight forward, but let me see if I have this right:
The purpose of the shims is to CREATE an interference fit, so if done properly, heating the receiver will be required.
Is it "safer" to freeze the barrel instead of heating the receiver?
I was reading another thread where the feed ramps did not smoothly align between the receiver and barrel. Can shimming/bedding mess up the feed ramp alignment?
Even with a thermal fit, folks are still shimming/bedding their barrels and seeing improvements in accuracy? Or is this just for non-thermal fit barrels?
I think I want to try this. It looks pretty straight forward, but let me see if I have this right:
The purpose of the shims is to CREATE an interference fit, so if done properly, heating the receiver will be required.
Is it "safer" to freeze the barrel instead of heating the receiver?
I was reading another thread where the feed ramps did not smoothly align between the receiver and barrel. Can shimming/bedding mess up the feed ramp alignment?
shim only if the upper and extension are not already a thermal/interference fit.
correct, and you want to heat the receiver, since it has less mass, and expansion happens faster at less temperature change than contraction
generally no, but IMO one of the things I always do after everything is installed before adding any other components is give the feed ramps a good polish so they're nice and uniform with the receiver anyway.
@Naaman
Not an expert, nor a great shooter than can shoot the difference in some of these methods but maybe I can help.
A true thermal fit, mechanical interference, for the inch or so of material that overlaps when the barrel extension is inside the upper receiver can be achieved when the bore diameter is LESS than the extension diameter.
Tight uppers can be 0.9980" or so, and an oversize extension 1.0010". That would give a Thermal Fit at room temperature and probably any temperature the joint will be in use.
I doubt you would want any more than 0.0020" interference at room temperature.
Normal uppers will be just under 1.0000" and extension just over 1.0000". Might seem tight and not really be a Thermal FIT. Especially when the extension slips freely into the upper for half or more of the length of overlap.
Some of the barrels I have are small for the first 1/8 to 1/4 inch maybe to get started then larger for the rest of the length.
Loose fitting uppers that require shims or goop could be out of spec and OVER a 1.000" bore.
Or an extension less than 1.0000" that will be loose as a goose in a oversize upper.
You can find undersized tight fitting uppers, and/or an oversize extension by shopping brands, or get lucky.
Either buy brands reported to be a tight fit, or MEASURE what you have.
You may notice that few assemblers actually report real measurement data though. Most say (or show on youtube) that they have a Thermal Fit because it was tight to assemble and had to apply some HEAT to get the parts together.
The NEED to shim (steel, goop, or spray-on) should be based on accurate measurement data. The amount of heat needed to expand 7075 aluminum alloy and slip tight fitting parts together can be calculated from actual measurements or by getting the aluminum 'Hot Enough'.
7075 aluminum used for uppers will have a thermal expansion/contraction of 11 to 13 microinches per degree F.
Raising the temperature of an upper
100F above room temperature (heat from 70F to 170F) will expand the aluminum about 0.0012",
150F rise (to 220F) about 0.0018",
200F rise (to 270F) about 0.0024",
These temperatures should be SAFE for anodized aluminum if the applied heat is even with no HOT spots.
Also remember that the extension goes in PAST the threaded NOSE about 0.6".
Heating just the thin nose might not be enough.
250F rise ( to 320F) about 0.0030" expansion, might be a little too hot.
Stainless steel used for rifle barrels will have a thermal expansion/contraction of 7 to 9 microinches per degree F.
(carbon steel barrels a little less)
Cooling the barrel by
30F (from 70F to 40F) will shrink the O.D. by about 0.00024".
70F (from 70 to 0F) will shrink the O.D. by about 0.00056".
Expansion/contraction will start to dissipate as soon as the heat/cold is removed.
You can see it would be difficult to get over 0.003" from heat/cold.
A full wrap of 0.001" shim stock would give 0.002" extra interference.
Needing 0.002" shim stock for an extra 0.004" interference means you got junk
I think, getting 0.002" expansion, and 0.0004" contraction would be a reasonable LIMIT (if you work fast).
A final interference of 0.001" to 0.002" should be plenty.
Having too much interference might cause the aluminum to YIELD (hoop stress beyond yield) and permanently expand and maybe distort the upper.
How much shimming might be needed really depends on KNOWING O.D. and I.D. before you start.
I'm going to add that rapid fire, or long strings could heat up (loosen) the joint and single shot hunting in the middle of winter could tighten the joint. Something to consider before making a final decision on how much interference you want.
Not an expert, nor a great shooter than can shoot the difference in some of these methods but maybe I can help.
A true thermal fit, mechanical interference, for the inch or so of material that overlaps when the barrel extension is inside the upper receiver can be achieved when the bore diameter is LESS than the extension diameter.
Tight uppers can be 0.9980" or so, and an oversize extension 1.0010". That would give a Thermal Fit at room temperature and probably any temperature the joint will be in use.
I doubt you would want any more than 0.0020" interference at room temperature.
Normal uppers will be just under 1.0000" and extension just over 1.0000". Might seem tight and not really be a Thermal FIT. Especially when the extension slips freely into the upper for half or more of the length of overlap.
Some of the barrels I have are small for the first 1/8 to 1/4 inch maybe to get started then larger for the rest of the length.
Loose fitting uppers that require shims or goop could be out of spec and OVER a 1.000" bore.
Or an extension less than 1.0000" that will be loose as a goose in a oversize upper.
You can find undersized tight fitting uppers, and/or an oversize extension by shopping brands, or get lucky.
Either buy brands reported to be a tight fit, or MEASURE what you have.
You may notice that few assemblers actually report real measurement data though. Most say (or show on youtube) that they have a Thermal Fit because it was tight to assemble and had to apply some HEAT to get the parts together.
The NEED to shim (steel, goop, or spray-on) should be based on accurate measurement data. The amount of heat needed to expand 7075 aluminum alloy and slip tight fitting parts together can be calculated from actual measurements or by getting the aluminum 'Hot Enough'.
7075 aluminum used for uppers will have a thermal expansion/contraction of 11 to 13 microinches per degree F.
Raising the temperature of an upper
100F above room temperature (heat from 70F to 170F) will expand the aluminum about 0.0012",
150F rise (to 220F) about 0.0018",
200F rise (to 270F) about 0.0024",
These temperatures should be SAFE for anodized aluminum if the applied heat is even with no HOT spots.
Also remember that the extension goes in PAST the threaded NOSE about 0.6".
Heating just the thin nose might not be enough.
250F rise ( to 320F) about 0.0030" expansion, might be a little too hot.
Stainless steel used for rifle barrels will have a thermal expansion/contraction of 7 to 9 microinches per degree F.
(carbon steel barrels a little less)
Cooling the barrel by
30F (from 70F to 40F) will shrink the O.D. by about 0.00024".
70F (from 70 to 0F) will shrink the O.D. by about 0.00056".
Expansion/contraction will start to dissipate as soon as the heat/cold is removed.
You can see it would be difficult to get over 0.003" from heat/cold.
A full wrap of 0.001" shim stock would give 0.002" extra interference.
Needing 0.002" shim stock for an extra 0.004" interference means you got junk
I think, getting 0.002" expansion, and 0.0004" contraction would be a reasonable LIMIT (if you work fast).
A final interference of 0.001" to 0.002" should be plenty.
Having too much interference might cause the aluminum to YIELD (hoop stress beyond yield) and permanently expand and maybe distort the upper.
How much shimming might be needed really depends on KNOWING O.D. and I.D. before you start.
I'm going to add that rapid fire, or long strings could heat up (loosen) the joint and single shot hunting in the middle of winter could tighten the joint. Something to consider before making a final decision on how much interference you want.
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One trick for cooling the barrel extension is to use dry ice. Your typical freezer is around 0°F, dry ice is -109°F, which will get you close to 0.0015" contraction. It's usually available at most major grocery stores and doesn't cost much for the amount needed. There are some tricks and warnings if using dry ice..@Naaman
Not an expert, nor a great shooter than can shoot the difference in some of these methods but maybe I can help.
A true thermal fit, mechanical interference, for the inch or so of material that overlaps when the barrel extension is inside the upper receiver can be achieved when the bore diameter is LESS than the extension diameter.
Tight uppers can be 0.9980" or so, and an oversize extension 1.0010". That would give a Thermal Fit at room temperature and probably any temperature the joint will be in use.
I doubt you would want any more than 0.0020" interference at room temperature.
Normal uppers will be just under 1.0000" and extension just over 1.0000". Might seem tight and not really be a Thermal FIT. Especially when the extension slips freely into the upper for half or more of the length of overlap.
Some of the barrels I have are small for the first 1/8 to 1/4 inch maybe to get started then larger for the rest of the length.
Loose fitting uppers that require shims or goop could be out of spec and OVER a 1.000" bore.
Or an extension less than 1.0000" that will be loose as a goose in a oversize upper.
You can find undersized tight fitting uppers, and/or an oversize extension by shopping brands, or get lucky.
Either buy brands reported to be a tight fit, or MEASURE what you have.
You may notice that few assemblers actually report real measurement data though. Most say (or show on youtube) that they have a Thermal Fit because it was tight to assemble and had to apply some HEAT to get the parts together.
The NEED to shim (steel, goop, or spray-on) should be based on accurate measurement data. The amount of heat needed to expand 7075 aluminum alloy and slip tight fitting parts together can be calculated from actual measurements or by getting the aluminum 'Hot Enough'.
7075 aluminum used for uppers will have a thermal expansion/contraction of 11 to 13 microinches per degree F.
Raising the temperature of an upper
100F above room temperature (heat from 70F to 170F) will expand the aluminum about 0.0012",
150F rise (to 220F) about 0.0018",
200F rise (to 270F) about 0.0024",
These temperatures should be SAFE for anodized aluminum if the applied heat is even with no HOT spots.
Also remember that the extension goes in PAST the threaded NOSE about 0.6".
Heating just the thin nose might not be enough.
250F rise ( to 320F) about 0.0030" expansion, might be a little too hot.
Stainless steel used for rifle barrels will have a thermal expansion/contraction of 7 to 9 microinches per degree F.
(carbon steel barrels a little less)
Cooling the barrel by
30F (from 70F to 40F) will shrink the O.D. by about 0.00024".
70F (from 70 to 0F) will shrink the O.D. by about 0.00056".
Expansion/contraction will start to dissipate as soon as the heat/cold is removed.
You can see it would be difficult to get over 0.003" from heat/cold.
A full wrap of 0.001" shim stock would give 0.002" extra interference.
Needing 0.002" shim stock for an extra 0.004" interference means you got junk
How much shimming might be needed really depends on KNOWING O.D. and I.D. before you start.
I think, getting 0.002" expansion, and 0.0004" contraction would be a reasonable LIMIT (if you work fast).
A final interference of 0.001" to 0.002" should be plenty.
Having too much interference might cause the aluminum to YIELD (hoop stress beyond yield) and permanently expand and maybe distort the upper.
Dip the receiver in boiling water, extension end of barrel in dry ice would do it.
I use a freezer and the oven.
I use a freezer and the oven.
Don’t get lost in all the technical stuff.
If you have a loose fitting upper/barrel, try to heat the upper and get a piece of shim stock in there, if you have trouble after a couple tries and now your out of shim stock, just gob some 620 loctite in the upper and on the barrel and shove them together and spin the barrel nut on, then move on with life. Like others have stated, the stuff seems very durable and will do just as good, some high end rifle companies seem to think so anyway.
I used 609 lock tite on a loose barrel extension into the receiver. It did cut group size considerably, 3 moa to 1.5, and significantly improved poa/poi between different loads.
(As a side note, cutting off the loose bore where it was threaded from factory made the greatest accuracy improvement. 7moa to 3 moa.)
At a later time I attempted to remove the barrel without heat. It was almost impossible without damaging stuff.
But my questions, Does the 609 or 620 loctite expand? Or just nearly permanently glue it in place?
And if so, is a thermal fit necessary? Or I guess the thermal fit would remain tight if the gun is hot as opposed to a cold fit would loosen with heat?
(As a side note, cutting off the loose bore where it was threaded from factory made the greatest accuracy improvement. 7moa to 3 moa.)
At a later time I attempted to remove the barrel without heat. It was almost impossible without damaging stuff.
But my questions, Does the 609 or 620 loctite expand? Or just nearly permanently glue it in place?
And if so, is a thermal fit necessary? Or I guess the thermal fit would remain tight if the gun is hot as opposed to a cold fit would loosen with heat?
It does expand, 609 and 620 expand to different ratios. But either is plenty for the void one might see in this process. 620 has a higher heat rating.
I think it expands causing a thermal fit if you will. A little heat on the receiver and the loctited barrel can be tapped out with a wooden dowel but cannot be removed otherwise. The loctite doesn’t bond to the receiver at all. It only seems to bond to the barrel extension. Loctite 609
Cool thanks. Yeah it was tight, the locktite all remained attached to the barrel extension, and came free from the upper receiver. Albeit tight. Heat did get it free but I had to try with no heat. Impressive stuff!
The lighter ft-lbs torque spec Joe Carlos was talking about were for heavy barrels, while the higher torque specs were for lighter, govt profile M16A2 barrels if I remember his findings correctly. Harmonic resonance dampening seemed to work better that way.
This is one of the reasons I do barrel nut torque spec by feel, not by an arbitrary spec. Different mass uppers and barrels work together in different ways. I have a Lilja Wasp lightweight profile 16” Grendel that is just a hummer, whereas you can have heavy profile barrels that are harder to get to shoot, though this is usually opposite of the anecdote I’m highlighting to make a point.
In the Aerospace sector during a flight test program, wing, tailplane, and structural buffeting has to be limited into acceptable nodes throughout the flight regime. Destructive vibrational effects on subcomponents also have to be ground and flight-tested, because they can cause loss of control or system failures at different speeds and air densities.
The F-14’s hydraulic lines were separated based on lessons learned from the F-105 (which had closely-located redundant hydraulic lines that could be taken out from battle damage, as seen over North Vietnam). The problem with the initial F-14 hydraulic line arrangement was that they were mirrored, so they acted like a tuning fork and hit such high resonant frequency nodes that they ruptured, causing the loss of Tomcat #1 BuNo 157980 on its 2nd test flight.
There are a lot of harmonic dampening techniques that are done on aircraft, much of which involves adhesives, torque specs on fasteners, interlocking structural layers of the bulkheads, longerons, stringers, and skin to form a monocoque structure. Composites do better than aluminum and titanium for a lot of that.
We often think of a free-floated barrel like a tuning fork in its relationship to the action, but we really want the least amount of harmonic behavior, and whatever vibration there is, we want it to be consistent shot-to-shot. If the barrel and upper receiver don’t behave as a single unit, then the barrel resonates differently. Slop between the extension and upper does not support consistent shot-to-shot behavior.
Since the steel mass of a barrel is so different than the lightweight aluminum mass of the upper, there is a lot of opportunity for harmonic inconsistency. Attaching scope mounts with heavy optics to the upper changes the dynamic resonance of the upper relative to the barrel as well.
I thought I remembered the torque spec he listed in the article but you are correct in that he was talking about a very specific type of barrel/rifle combination.
If I ever get a wild hair and want to drive myself nuts (like when I owned a NECO Run-out gauge) I might test this. Of course, like the mentioned NECO gauge, it would apply to that particular upper and barrel sorta kinda. After looking in the mirror, I have enough grey hair.
Good point though and I'll more than likely test it on one of my "accuracy" AR's because I'm nuts.
One thing mentioned about bipod loading and its effects on strain against the upper and barrel:
If you want to test the mechanical accuracy of an AR-15 or AR-10, using one of those bull bags tucked up against the front of the magazine well, instead of out front, helps with this. Tom Beckstrand wrote a great article about this a few years back, in addition to the effects of pre-ignition vibrations that affect precision in the Stoner platforms. This doesn’t help you in practical use of the rifle if you rely on a bipod in shooting positions though, other than showing you what the rifle is actually capable of.
Biggest effects on precision seemed to be the hammer swing and its vertical displacement of the receiver set due to spring tension that is perpendicular to the bore axis. Hammer mass and firing pin contact with the internal geometry of the FP channel were others.
The Alexander Arms tri-lobal firing pin seems to have been an effort to address this, as well as particles and debris interfering with Firing Pin travel inside the bolt.
Light hammers can go a long way in reducing group sizes, but they don’t do well with durability unless you get into exotic alloys and processes.
If you want to test the mechanical accuracy of an AR-15 or AR-10, using one of those bull bags tucked up against the front of the magazine well, instead of out front, helps with this. Tom Beckstrand wrote a great article about this a few years back, in addition to the effects of pre-ignition vibrations that affect precision in the Stoner platforms. This doesn’t help you in practical use of the rifle if you rely on a bipod in shooting positions though, other than showing you what the rifle is actually capable of.
Biggest effects on precision seemed to be the hammer swing and its vertical displacement of the receiver set due to spring tension that is perpendicular to the bore axis. Hammer mass and firing pin contact with the internal geometry of the FP channel were others.
The Alexander Arms tri-lobal firing pin seems to have been an effort to address this, as well as particles and debris interfering with Firing Pin travel inside the bolt.
Light hammers can go a long way in reducing group sizes, but they don’t do well with durability unless you get into exotic alloys and processes.
The brown stuff? that is some nasty glue! I've used it on radiator hose connections and thermostat housings.
Many issues with these videos! Seekins is similar to the AP's enhanced upper (good news) as both have the barrel nut with male threads! In other news if you are not going full circle with a so-called shim it will torque it in the direction where there is no shim! And yes, to much slack is not going to be taken up by the barrel nut! With the barrel extension NOT making contact with the upper before the nut is torqued it's best to shim between the extension and the front of the threads! There is where headspacing can be an issue! If lapping an upper is needed, send it back! Mating an upper with the barrel that has a tight-fitting extension can be an issue with male threaded upper as being the weaker link. Best way to do it is like press fitting wrist pins onto a connecting rod. Freeze the pin and heat the rod. In this case freeze the barrel and heat the upper. If the extension isn't squared with the front of the upper (threads) (if machining is done wrong) you will be placing excess pressure between the indexing pin and the upper. The pin needs to be a guide, not a forced area! Pin shouldn't touch the back of the slot!
It takes a lot more than a hairdryer to heat it enough. If it works for wrist pins and rods it will work for barrels and receivers.
Practically that guy is right. Technically he is wrong.
His freezer (18 degrees f) would have smallered the barrel extension by 0.0003”.
Hair dryer (130degrees f) would have largered the receiver by 0.0005”
A heat gun can get the upper hot enough to get the desired thermal fit.
And there are colder things than household freezer.
Ah, now to hear about all of SH’s ex-wives’ hearts
For a good freeze it will take 12+ hours depending on temp of the complete barrel. A heated vat is where we heated the rods (laying on the cover) for 6+ hours, so the receiver for a good even and not fast heat up could go in the oven for 4-6 hours @ 200*. Lube the receiver as the extension might make the lube stick to it. It will work.
I did a bunch of dynamic shooting over the weekend and saw a lot of POI shifts when shooting off barricades vs bipod, etc. I would assume bedding a barrel extension into a solid receiver with an already tight but not thermo fit ought to help see some of this go away? Seems like the Brownells guys were dismissing the need for people to bed the receiver in order to get accuracy, but I feel like handguard to barrel to receiver flex under different pressures is talked about very rarely.
I tested this several years ago and posted the results on here and I don’t believe it’s a thing when using good receivers.
The test I did was with a VLTOR MUR1A receiver, Seekins SP3R forend, and Craddock 224V barrel that was stupid accurate. I didn’t have a way to scientifically measure the pressure that was being put on it in either direction but I shot groups loading harder into the bipod, digging the legs in one side at a time and panning it to put more pressure towards one side and I couldn’t get it to shift off of a 1” target pasty. There was a very subtle shift at times but not anything that would make any real difference and I had to push extremely hard to see any deviation.
That was a thick walled 7075 receiver though, in a standard receiver and 6061 the results might be different but I don’t use those so I honestly don’t care.
Good to hear. I'm working on upgrading from a mil spec receiver (can't remember if this one is the BCM or an Anderson forged) to a Wilson Combat billet .458 socom reciever that's significantly more beefy through the area that holds the barrel extension, at least on one side. I wish we could just ditch dust covers and reenforce the area under the ejection port.
What you're seeing is the receiver flexing behind the thread for the barrel nut. Bedding the extension won't help.
The barrel extension sits further into the receiver than that point and if you take up any gap around then outside of the barrel and inside of the receiver it absolutely will make it more rigid and help to mitigate that flex.
It’s a simple geometry issue. There is not a lot of meat behind the receiver thread on a milspec upper. That area flexes with the barrel nut when pressure is applied to the hand guard. This is common knowledge. It doesn’t matter how deep the extension goes. That is the flexi point.
That's why I went with the Wilson. The VLTOR and Noveske Gen 3s are all sold out, and experience has taught me that I want a forward assist. A lot of the beefier upper receivers with more meat in the walls and around the barrel extension's termination point don't have forward assist.
But back to the point, I'm hoping once I get a barrel upgrade and bed it into a heavier receiver with less flex, that I should see less POI shift.
Common knowledge… do you have data? Or computer models?
I believe that when SF tried updating their Mk12s to KAC URX 4s from the PRIs that they experienced POI shifts from force applied to the handguard that they did not observe with the PRIs. The thought was that it was the integral barrel nut moving the barrel, not that the receiver was flexing.
I could definitely understand a POI shift from forces on the handguard if the receiver face is not trued and there is room for the receiver extension to wiggle. Even with a torqued down barrel nut, if the receiver face is not true, there could easily be a rocking movement.
For receiver flex, are you referring to just the 90 degree section from the threads to the rest of the receiver? Because the area behind that bend has a lot of rigidity to it. But the 90 degree bend between threads and receiver is not a whole lot of material or structural geometry. But as mentioned, the receiver extension extends beyond that 90 degree area at the threads. If there is slop between the barrel extension and receiver, it could understandably flex the threads. But if there is a solid fit, the barrel extension would have to overcome a lot of rigidity in the design there.
I’m not saying you’re wrong, just that it doesn’t seem very plausible without seeing some evidence. If you could elaborate.
Aero enhanced is a cost effective option. They did add meat @ the front. Can confirm though, Aero are not thermal fit.
Hm I think I have a new Gen 3 stripped upper chilling in the basement somewhere. Seen VLTOR in stock recently actually.
Maybe I didn't look hard enough, lol.
Keeping the thread on topic though - what is the consensus?
- If it's thermo fit, you're G2G with just thermo fitting it
- If it's a good fit that you can slip in, #620 Loctite (honorable mention to #609)
- If it's a poor fit, chuck it, or resort to shims and loctite?
You can literally watch your POI shift and you load up your bipod...
Where does the barrel (extension) gets most of it's support?
That doesn’t isolate it to receiver flex. It could be the barrel/barrel extension moving.
If someone has some engineering software that can show a model of a 7075 aluminum receiver itself flexing when those loads are applied to the barrel nut, that would be great. Or if someone has a tried receiver face with thermal fit barrel experiencing the similar POI shift with n lbs of force applied to a handguard on a free float system.
I just haven’t seen either of those to conclude what you’re concluding.
A heavy walled upper, IMHO, is never a bad thing when you're trying to make the whole thing stiff and tight. I do not know how much trying to make an AR as stiff as possible helps but I do know no one would be satisfied with a wobbly bolt action. There is a whole lot more going on and a lot more pieces moving with an AR, so why not try to make them as stiff as possible? I'm mainly talking about an AR that you'll use as a target rifle but I'll also go to the trouble of stiffening up a self defense AR. I've never noticed any of my rifles malfunctioning due to this extra stuff done to them. Just my experiences.
I haven't seen that particular upper but I do use 2 other styles of a heavy walled upper. The Nordic Components upper doesn't use a dust cover or FA. It's as plain as a sheet of plywood but a thick and stiff girl. I prefer an Iron City Hatchet which does use a dust cover (no FA) but is still thick and a lot prettier. Ugly rifles may shoot good but damn.
Bedding/shimming/thermofit does not eliminate the shift.
I got 2nd place overall in Gas Gun PRS this year, and in talking with the guy that got 1st place (also a couple of the other guys in the finale gas gun squad) we see the same thing. 0.2-0.4 mils of elevation change in POI going from a bag to a bipod. We were mostly all using Aero enhanced (beefy barrel mount area), JP, or Seekins uppers, or something of that style. It was dead repeatable every time.
If you lock pretty much any AR-15 FF handguard into something like a tank trap-- like lock it down hard so it's wood-aluminum-wood with tension, it will shotgun pattern. If you torque the bipod left or right it will string shots left and right.
Here's the thing, the upper rail holds your optic, the upper receiver bore holds your barrel. ANY movement of the rail or the receiver bore is pretty much directly causing POI error. The barrel extension has something like .845" of "wheel base" in the upper. 0.2 mils over .845" is 0.000169". So whatever loading you do on the handguard needs to cause 1.7 TENTHS of deflection to the mouth of the upper. Not surprising that it happens.
We get spoiled with separated chassis/receiver setups in bolt actions with steel receivers-- Larger diameter (more rigid) larger clamping forces (more rigid, less slop), and steel on steel (more rigid), plus the chassis/stock and receiver are separated from each other. AR's are a different world.
Vltor makes a rail that attaches to the top rail. The other option is a monolithic upper.
This makes a lot of sense, and it's what I was seeing last week at a PRS match. I'm using a pseudo MK12 Mod 0 Clone.
Zeroed my gun on a bipod (would be torquing the barrel high)
On stage 1, was shooting off a bag, not loading into it much. Was alright, maybe a bit high, and adjusted toward center of the plate.
Next stage, PRS Skills, and was shooting consistently .2 low. Adjusted my dope to shoot .2 higher.
Next stage, tank traps. On this one I started loading into the bag more, and needed another .2. I was now adding .4-.5 to my og zero.
Next stage, I was on a less steady barricade, and couldn't load as much into the bag, and was shooting high.
Rinse repeat, and I was chasing my zero the whole day. This was using a PRI delta forearm with a full length top rail which I had locked down pretty tight the day before.
Seems like if I want to do more gas gun at matches, I'm going to have to do paper tests at 100 to see what affects POI, and either account for it based on position, or adjust my shooting discipline to minimize POI shift.
If you're right on the angular changes to the barrel's shift, I wonder if the gas tube can also end up touching stuff and screwing up free float as well? I've always thought it was weird that we talk free float handguards for accuracy so much, but almost no one talks about the fact that there's a tube attached to that runs through the receiver and seals up against the BCG.
I'm still going to lock down that barrel extension, even if it only nets me .05 MRAD instead of .2.
