Trim Length: How Crucial Are Those Last Two Numbers?

[Familydude]

Been perusing the threads. Didn’t find what I’m looking for. Please point me in the right direction if I missed it:

6GT Alpha Munitions OCD brass. SAMMI recommend trim length for 6GT is 1.7250. In past attempts to trim with my Lyman, and admittedly having little experience trimming, I have a few at the recommended length, but others are at 1.760, 65 and 80.

On the short side, I have some from 1.7220 to 1.7245.

Is there an acceptable range, or should I try and be spot on? How might consistency possibly be affected being dead on vs. within the possible range?

Barrel is a Proof pre-fit Competition Contour.

Thanks for any input.

 

[Baron23]

Been perusing the threads. Didn’t find what I’m looking for. Please point me in the right direction if I missed it:

6GT Alpha Munitions OCD brass. SAMMI recommend trim length for 6GT is 1.7250. In past attempts to trim with my Lyman, and admittedly having little experience trimming, I have a few at the recommended length, but others are at 1.760, 65 and 80.

On the short side, I have some from 1.7220 to 1.7245.

Is there an acceptable range, or should I try and be spot on? How might consistency possibly be affected being dead on vs. within the possible range?

Barrel is a Proof pre-fit Competition Contour.

Thanks for any input.

Look at the SAAMI spec drawing. Note the -.020 after the nominal case length.

https://saami.org/wp-content/uploads/2022/01/Public-Introduction-6mm-GT-2022-01-28.pdf

 

[Familydude]

Thank you. Had this printed out but wasn’t sure what I was looking at. Looks like the acceptable range is 1.705 to 1.725 if I’m reading it correctly. I don’t imagine the difference in the two would be enough to have a noticeable affect on consistency in a PRS setting, or am I wrong?

Sorry for the dumb questions.

 

[RegionRat]

If you are just starting out with precision loading/shooting, just do your best to keep the brass prep and loading work as consistent as you can.

In time, you should take the parameters you are wondering about, and explore them by checking the results on the target.

While you are starting out, it is often difficult to shoot the differences in certain parameters, but as time goes on and you improve you may find that you can find a difference.

If you study reamer prints versus the SAAMI specs, you will see there is some variation in how much margin they leave for case neck length. Brownell's carries little tools you can slip into a piece of brass and force into the chamber that helps put numbers on that length margin.

https://www.brownells.com/reloading/measuring-tools/case-length-gauges/sinclair-chamber-length-gage/

Don't worry to much about this yet, but if you are curious you can get one of those and measure exactly how long they cut your chamber. All this does, is tell you the margin above the max spec length. I don't recommend you let the brass go there. For example, if the chamber is at 1.732, and your brass is at 1.725, there is a difference of roughly 0.007"... but the headspace difference can still allow the neck to move forward.

For now, try not to let them go longer than the MAX (1.7250) on the specs, and you don't need to go all the way down to 1.705 but it isn't like hitting a wall where your tune stops working.

Some day, do a side by side and see if you can tell any difference at all between ones left at max versus ones cut to min. Remember, there are always statistics at play with groups and samples that can fool you, so it would take a strong contrast to hint at any real difference.

 

[DownhillFromHere]

Thank you. Had this printed out but wasn’t sure what I was looking at. Looks like the acceptable range is 1.705 to 1.725 if I’m reading it correctly. I don’t imagine the difference in the two would be enough to have a noticeable affect on consistency in a PRS setting, or am I wrong?

Sorry for the dumb questions.

Your trim length should be in the middle of the SAAMI spec range; e.g., 1.715 inches. With practice, you should be able to trim consistently and quickly to within plus or minus a couple of thousandths of your target length. Make sure you resize your brass before trimming, and annealing after a few firings is beneficial, if not required, for best case life and velocity consistency.

If you've got cases that are 1.760" or more... I wouldn't want to shoot them. A case that's too long could be precluded by the chamber from releasing the bullet as intended, possibly resulting in dangerous or catastrophic pressure spikes. Consistently far too short, and you get carbon buildup in front of the chamber which could raise pressures when correct-length rounds are fired.

 

[straightshooter1]

Been perusing the threads. Didn’t find what I’m looking for. Please point me in the right direction if I missed it:

6GT Alpha Munitions OCD brass. SAMMI recommend trim length for 6GT is 1.7250. In past attempts to trim with my Lyman, and admittedly having little experience trimming, I have a few at the recommended length, but others are at 1.760, 65 and 80.

On the short side, I have some from 1.7220 to 1.7245.

Is there an acceptable range, or should I try and be spot on? How might consistency possibly be affected being dead on vs. within the possible range?

Barrel is a Proof pre-fit Competition Contour.

Thanks for any input.

What I would say is most important is to trim to a consistent length, as there are a couple of things to consider, besides the SAAMI specs.

If trim length varies by quite a lot, one can have an issue with the carbon ring building up. Let's say most of your brass is trimmed to 1.725 and as you fire them where the carbon builds up in from of that. Then you chamber a cartridge that is 1.765 where the carbon ring has built up substantially, that carbon ring can cause the mouth of the case to pinch the bullet causing a spike in pressure that can be seen on a chronograph as significant increase in velocity. It's not necessary a safety hazard, but more of throwing your shot off because of that increased velocity. On the opposite side, if most of the cases are long, the shorter ones will not have this issue.

When a cartridge is being fired, the case begins to expand and the bullet begins to be released and as this process at its early stage the blow-by increases a lot when when the bearing surface clears the case mouth and chamber pressure rises rapidly. The timing of this is effected to a small degree for the difference in case's trim length. The effect is a small thing, so whether it's important to you or not depends on what discipline of shooting you do (like competitive bench rest shooting vs just hunting).

The trim length you decide to use can depend on what bullet you're using and how you intend to seat them. Trimming them long helps with the seating the bullets longer (an issue that can be important if one has a rather long freebore). But, you don't want to let them get too long so that there's room enough for variance between the freebore length and the case length.

I like to keep my cases .010 short of the maximum length of the freebore so there's not so much carbon ring surface to clean out AND I tend to load all my bullets long in a long freebore. Since consistency is very important to me, I trim my case to length every time after sizing.

Well, I hope this just gives you some ideas.

 

[Familydude]

I appreciate all of the informative responses. Thanks for taking the time to help out.

I’ll go with 1.725 for this next round since a majority are at that length. I’ll trim the remaining few longer ones to that. I’ll try the 1.715 to 1.720 next go round. Any casings shorter than this, I’ll set aside for now. Thankfully there aren’t too many.

Thanks again all.

 

[Baron23]

Thank you. Had this printed out but wasn’t sure what I was looking at. Looks like the acceptable range is 1.705 to 1.725 if I’m reading it correctly. I don’t imagine the difference in the two would be enough to have a noticeable affect on consistency in a PRS setting, or am I wrong?

Sorry for the dumb questions.

Not dumb, and imo, yes to your questions.

I too aim for .010” short of max to ensure smooth chambering and be not so short that I will build carbon in larger area of the chamber.

I was….still am…on my phone, hence the brevity. But you got some really good response above.

The reloading forum seems to have none of the BS we see elsewhere on the Hide…and tons of good solid advice and guidance….because it’s important to personal safety.

In that light, there are really very few “dumb questions”.

Best of luck.

 

[Familydude]

The reloading forum seems to have none of the BS we see elsewhere on the Hide…and tons of good solid advice and guidance….because it’s important to personal safety.

Amen.

 

[supercorndogs]

Concerning the last tow numbers .0000. I wouldn't expect a caliper to be accurate to the third and fourth zeros, thousandths and ten thousandths.

 

[Doom]

I'm assuming that the 1.760 and similar are actually 1.726, etc. The Lyman trimmer can be pretty consistent, usually less than .001" difference but if you are doing a heavy trim you may need to actually trim twice. Depending on technique the neck wall material may flair (thicken) at the cutter. when this happens you remove and flair, then repeat to get down to the correct size.

 

[Doom]

Amen.

Most of the time!

 

[Familydude]

I'm assuming that the 1.760 and similar are actually 1.726, etc. The Lyman trimmer can be pretty consistent, usually less than .001" difference but if you are doing a heavy trim you may need to actually trim twice. Depending on technique the neck wall material may flair (thicken) at the cutter. when this happens you remove and flair, then repeat to get down to the correct size.

Yes, thank you for catching my typo. 1.726, etc.
l just picked up an RCBS Pro Trimmer and a 3-way cutting head. The Lyman, coupled with my inexperience, was not consistent. Again, I’ll blame myself on that. Hoping the RCBS takes care of that once it’s set up. Thanks.

 

[DownhillFromHere]

Yes, thank you for catching my typo. 1.726, etc.
l just picked up an RCBS Power Trimmer and a 3-way cutting head. The Lyman, coupled with my inexperience, was not consistent. Again, I’ll blame myself on that. Hoping the RCBS takes care of that once it’s set up. Thanks.

Consistent trimming takes practice, no matter what trimmer you use. You'll literally develop a feel for it.

 

[supercorndogs]

I will mash them throught a WFT trimmer all day and not measure any of them. When i have had an accuracy problem. I have never traced it back to my trimming. I think a lot people go looking for little tricks and sorting the minutiae when they should be focusing on the basics. I see lots of suggested processes I bet add nothing but time reloading for most shooters.

 

[Familydude]

I’ve reached the stage in my life where I’ve got lots of spare time, even with family, car/house maintenance and volunteer stuff. I don’t mind spending a few extra minutes trimming brass. Gives me something more to do and I figure since the cutter will trim, chamfer and debur all at the same time, I’ll end up having spent roughly the same time on this step as I do chamfering and deburring in two steps. Certainly looking for consistency, but also like to be within SAMMI specs. Just a preference.

Definitely practicing shooting fundamentals in addition to this. Quite a bit.

 

[DownhillFromHere]

I’ve reached the stage in my life where I’ve got lots of spare time, even with family, car/house maintenance and volunteer stuff. I don’t mind spending a few extra minutes trimming brass. Gives me something more to do and I figure since the cutter will trim, chamfer and debur all at the same time, I’ll end up having spent roughly the same time on this step as I do chamfering and deburring in two steps. Certainly looking for consistency, but also like to be within SAMMI specs. Just a preference.

Definitely practicing shooting fundamentals in addition to this. Quite a bit.

Yeah, I'm retired too, so I have lots of time to fiddle with reloading. But I've been doing it for over half a century and I regard it with distinct distaste. Of shotshell, handgun, and rifle reloading, precision rifle is by far the most fiddly, persnickety, time-sucking, expense-laden chore associated with any shooting sport. I miss the simplicity of shotshell reloading for sure.... put in something close to the right amount of an appropriate powder and shot with the right wad and either the plastic hull holds together well enough to keep the shot from falling out or it doesn't. Biggest aggravation is spilling shot on the floor....

 

[Bobby Ball]

Curious to hear how you like the 3-way trimmer. I just ordered one and I'm waiting for it now. I have the rcbs trimmer also. I have love hate issues with it. I bought the 3-way mostly because I was neck turning separately and it was a pain and I also felt with my manual Sinclair doing it by hand it was difficult to maintain a consistent alignment and therefore consistent cut. I also found I was getting a fair amount of brass flow and when I had to case trim again the walls of the neck were getting so thick that it would leave a ridiculous ridge on the outside of the neck that the outside chamfer wouldn't take off. I had issues with learning to do accurate length trimming when I started, (and let me clarify when I say I started I mean only several months ago). But I found that my loads wouldn't even chamber if I was even remotely over the stated case length. It was a custom chamber that was cut to very tight tolerances. So when I read your initial post I thought Jesus he can still get it in the chamber? But your latest post made a lot more sense. The rcbs does have the adjuster that is supposedly a thousandth a mark and it probably is, but even then it's not a set it and forget it deal. It still takes a special touch. I also found I was not getting square cuts so every time I get about halfway through cutting I'll push down on the lever holding the case in place and let the cutter spin it occasionally and that seems to help get a more concentric cut. It will also help to get it aligned before you start the cut. FINALLY... There is a weird back cut bevel between the back of the spindle mount and the adjustment dials that makes it appear to still have a gap between the two pieces making you think that you have to keep cutting or pushing harder to close that gap. Don't let it fool you and just take a lot of time up front and measure the length a bunch of times in the beginning and it will save you a lot of headaches. Sorry that was so long winded.

 

[Familydude]

The 3-way cutter has been great so far. Went through a couple pieces of test brass but ultimately got all three blades and trim length set just right. Trim length has stayed spot on. 250 casings done to this point. Use the same method as you when it comes to centering the casing with the guide. Slightly release pressure, couple quick turns to better center casing, trim, chamfer, deburr. Gets it all done relatively quickly.

 

[DeathBeforeDismount]

Bite the bullet and get yourself a henderson. Never have to worry about trimming/chamfering/deburring again. All of my GT cases are right at 1.721 , and the henderson keeps them within a thou or so. Its the fastest, easiest and most consistent way to trim, bar none. What used to be the most hated, painfull and tedious part of reloading is now the fastest and easiest.

 

[Familydude]

Henderson is the cat’s meow. Have also looked at Giraud. I’ll probably eventually work up to one of the two. I’ve “bitten the bullet” quite a bit lately. If I don’t slow down I’m going to be selling citrus on the side of the road.

 

[John Glidewell]

I will stick with my 21st century mini lathe with 3 way trimmer.

 

[TSULLI]

Henderson is the cat’s meow.

Definitely on my list for upgrades...

 

[Haney]

If you desire to know Exactly what your trim length should be you can use these gauges . Trimming too short leads to problems .

https://www.brownells.com/reloading/measuring-tools/case-length-gauges/sinclair-chamber-length-gage/

 

[DaveBG15]

If you desire to know Exactly what your trim length should be you can use these gauges . Trimming too short leads to problems .

https://www.brownells.com/reloading/measuring-tools/case-length-gauges/sinclair-chamber-length-gage/

How does trimming to short causes problems? I am helping a friend with load development and he trimmed his cases below SAAMI specs by .004. 7MM PRC 2.280 -20 his case 2.256.

 

[straightshooter1]

Obviously, it's not ideal. But he can still use them depending on the bullet and as long as seating allows for the neck to be holding enough bearing surface. After FL sizing, the case OAL will likely grow by that amount either in the first sizing or the next.

 

[Bobby Ball]

I've done that same thing before with 300 prc cases, and they have shot fine. Like Straight shooter mentioned above they will probably stretch that distance after the resize. I'm on like 11 or 12 resizes on some of my adg 300prc brass and I usually have to trim every 3rd firing or so. Depending on the load, sometimes less sometimes more.

 

[918v]

OP:

Did you mix the long ones with the short ones? How did you arrive at this huge range of case length?

 

[Baron23]

How does trimming to short causes problems? I am helping a friend with load development and he trimmed his cases below SAAMI specs by .004. 7MM PRC 2.280 -20 his case 2.256.

As far as I know its mostly a matter of not allowing carbon to build up short of the end of the chamber's neck area. More area for potential carbon ring and perhaps issues of carbon build up making chambering of proper length case difficult

 

[Bobby Ball]

Fwiw I use a rcbs hand trimmer, and I still have a heck of a time getting everything within a thou unless I'm super careful and stop and measure multiple times. It's actually a little bit annoying. Well not even a little bit, it's really f-ing annoying. I can easily see how someone could end up 4 thou short.

 

[JerkyFreak]

If 0.0005" matter to you and you are on a budget, get the Lee cutter, lockstud and cartridge specific pilot. It is the easiest method to cut every case the same length. Also the simplest method to use. Only downside is you have to champher and deburr seperately

 

[DaveBG15]

If 0.0005" matter to you and you are on a budget, get the Lee cutter, lockstud and cartridge specific pilot. It is the easiest method to cut every case the same length. Also the simplest method to use. Only downside is you have to champher and deburr seperately

He has the Frankford Arsenal platinum case trim and prep center. I just think he didn't set it up correctly. I have a little crow WFT case trimmer. I love it! I chuck it up in my lathe and it works great.

​

 

[DaveBG15]

OP:

Did you mix the long ones with the short ones? How did you arrive at this huge range of case length?

No, I have not done anything yet. I was checking everything out for him. He is just starting out reloading and did not ask questions! He is 22 , the same age that I started out. I am now 66 (WOW) There are not many kids out there doing this kind of thing. In a way he kind of reminds me of myself when I was his age. Just waiting to talk to him to see what he wants do do. I have always trimmed to mid SAAMI specs.

 

[Rocketvapor]

Watch your neck for carbon and length.

 

[Seth W]

The 3rd number is thousandths. (.300) a decent dial caliper should have no problems reading that to the single thousandth. The 4th number is ten thousandths. (.3005) Dial calipers are not designed to read that, you need a micrometer. In any case, going finer than a single thousandth I wouldn't imagine would be necessary.

 

[DaveBG15]

FYI