Gunsmithing Question on pad under barrel

[pinzmann]

How important is it to have a bedding pad under the barrel just in front of the recoil lug?
This is on a Tac338 action with 29in barrel, 1.25" breach end. 338 Lapua mag. The action is single shot so it's bedded completely under the action. Manners stock.

 

[Mgordon]

Re: Question on pad under barrel

You will get different answers for different builders.

I'm going to do some testing shortly to figure out what our official procedure is going to be.

Mark

 

[turbo54]

Re: Question on pad under barrel

Mark, I've bedded one of my rifles (that shoots 3/8 moa) 5 times, trying it both ways. Other than a zero change each time, I wasn't able to see or shoot the difference.

Uncontrolled variable was "optimal load". I did not redo load development every time, I just used the load I knew that rifle shot great.

Personally, I prefer the pad... ...but for no good, legitimate reason whatsoever.

 

[MikeeBooshay]

Re: Question on pad under barrel

I'm building a 338LM right now also. After whittling out the stock inlet ( Rem LA ) to accomodate the Big Surgeon, I kept looking at it going Hmmmmmm......that 30" of big barrel, is sure gonna put a bind on the action threads. I didn't like the idea of a bedding area ahead of the recoil lug, what to do????

So, I am going to use a bedding block instead. It's a test bed for a LR BR project, and weight is no object. I will be sinking the block into the stock just fwd of the recoil lug, and floating everything not in the block. Pics later.

MikeE

 

[Keith Johns]

Re: Question on pad under barrel

I base my decision on the barrel. If it has a large, heavy cylinder that is just begging to have a pad under it, I put about a 1" pad under it right infront of the lug. And I do not tape off the front of the lug. So its solid front to back.

We pretty much only put pads under barrel contours <span style="font-weight: bold">larger</span> than Remington Varminter or Sendero. But if it's a really long barrel(28-30") I might put a 1/2" pad regardless of contour.

 

[Straight Shooter]

Re: Question on pad under barrel

Our mentors have me convinced that there should be nothing under the barrel but air on any bolt action rifle. Otherwise, any pressure of the rifle resting on the stock is transmitted with leverage right into the barrel through the bedding pad. We bed only the action.

If you are getting zero changes from redoing your bedding there must be some stresses in your bedding.

I also agree with the idea on big barrels the very best method is a nice bedding block and floating everything else.

 

[Mgordon]

Re: Question on pad under barrel

I'm not trying to fuel the fire, but here are the thoughts I struggle with.

1- AR-15's have less threaded tennon than a R700, and they seem to do really well. Yes the barrels are lighter, but proportionally speaking they may have the same weight load.

2- I dont buy the fact that the barrel is getting enough support to make a difference by adding 1" of bedding under the chamber. So now you have 29" of unsupported barrel instead of 30".

3- There are enough variables that can change after bedding that would not put the barrel back in the bedding the exact same way. What if a different smith took the barrel off for threading, then did not torque it down to the same spec? What if the barreled action got painted with something thicker than Cerakote, is it going to fit in there perfectly.

These are just some of the questions that I have and I'm going to work on getting answers to by my own testing. I know lots of the top shops out there are doing it. But I have never been one to jump on the bandwagon unless I believe in my heart that its the best way. Same thing with bedding epoxy, parts manufactures and so on.

Here is a challenge to the thinkers out there. Come up with a scientific test that is not flawed and would produce conclusive data, and I will do the testing. Break it down Barny style, and I will do it!

Mark

 

[Mgordon]

Re: Question on pad under barrel

One more thought, as chassis systems gain popularity, many shooters appreciate the benefits of these systems. Most would argue that they are just as good as a properly bedded rifle. I cannot think of one chassis system that uses any type of bedding block system under the barrel. Why would they, it would be impossible to do so.

So are these shooters who run chassis systems (that are not inducing stress on the action) missing out on accuracy potential. I'm sure the chassis manufacturer would definitely say not.

So it just does not make sense that adding a barrel pad would increase accuracy. The only thing I can see a bedding pad potentially do is dampen harmonics a little possibly increasing an accuracy node. I do not subscribe to the idea that barrels shoot better with 1 less inch of unsupported barrel.

Mark

 

[Keith Johns]

Re: Question on pad under barrel

In terms of support, I look at it like this...

Adding a 1" pad, you look at it like you have 29" of unsupported barrel compared to 30". I look at it like you now have 1.5" of bedding infront of the front action screw, rather than only 1/2". (recoil lug doesnt count. taped off on bottom)
Having 1.5" of support infront of the front screw, rather than just .5" I care about.

So I've tripled or doubled the support in the area I care about(infront of the front action screw)


And does the pad HELP accuracy? Of course not. I think it just keeps things more stable over time and abuse.
Does it hurt accuracy if done on a nice thick barrel? Obviously not because some of the most accurate rifles Ive seen had pads.


I've learned a lot and changed some of my methods in the past year. Im all ears for facts and info on bedding methods.
Again I dont think rifles NEED a pad, but I like doing it on big barrels.
If anyone can show a reason NOT to do it on heavy barrels, I'll stop.

 

[vonbalkenbush]

Re: Question on pad under barrel

Always wanted to know the answer to this and I'm not sure there is a statistically significant one out there. I bed a 1" pad on all my 700's, that's how it was taught to me and they all shoot great. I just bedded my first Bighorn which has an extra .300" of thread tenon, so I did a 1/2" pad in front of the lug instead. It's shoot's lights out.

If you look at it from a physics point of view, the torque applied to the action from the barrel hanging off is (length of barrel)x(weight of barrel), the length being from the front of the recoil lug (if no pad) or the end of the bedding pad (if there is a pad). The weight being from the barrel's Center of Gravity forward of the lug or pad. This seems straight forward and would tell you not to add a pad if just looking at the torque applied issue, but if you look at the chamber end (where the pad is) like Keith pointed out you have just effectively doubled or tripled the surface area the torque is being dispersed through. So now you have effectively decreased the binding torque on the action by 2-3 times by adding a pad. Now whether this actually translates into smaller groups down range is anybody's guess, but my thoughts are that having less stress dispersed through the tennon area and the rest of the action cannot do anything but help accuracy.

-SBS

 

[RTK]

Re: Question on pad under barrel

Thinking about it the pad could actually act a a fulcrum lever???

 

[Keith Johns]

Re: Question on pad under barrel

Its probably just personal preference for the most part.

And I could see how one beds affecting his opinion on this.

I bed the front of the lug solid when I can. Only taping off the sides and bottom. Where as most people tape off the front too.
I have seen very positive things so far from doing it. And no negatives from a small pad. Just keep the pad on the thick cylinder and away from the swoop. Rarely more than 1" long.


Only thing that worries me about a pad, is the barrel heating up REALLY hot and expanding on the pad. Don't know if this is a legit concern or not.

 

[300sniper]

Re: Question on pad under barrel

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Keith Johns</div><div class="ubbcode-body">Only thing that worries me about a pad, is the barrel heating up REALLY hot and expanding on the pad. Don't know if this is a legit concern or not.
</div></div>


think about this, the tenon is torqued into the receiver, that front ring is going to be expanding with the tenon with heat and pressure.

the only downside i see with bedding a pad in front of the lug is having to redo it at every barrel swap. but i also have a 31" barrel plus a heavy brake hanging off my receiver with no bedding and it shoots awesome.

 

[Keith Johns]

Re: Question on pad under barrel

Yep. I can def see how one would argue against a pad.

I just like em. And I'll need to see a reason to change before I don't like them anymore.

If a customer prefers one way or another it's fine by me.

 

[wnroscoe]

Re: Question on pad under barrel

Metal heats up and cools down, yes, even from just being in the sun, it moves.

When the bedding job was performed I bet the barrel was cool to the touch, 72ish degress maybe. Now, it's home on the range, pun there, and in the sun.

What does metal do when it heats up? It starts to move around. Measure a small peice of metal at room temperature. Now place it in the freezer for 10-15 minutes or longer, measure agian. Now, heat it up,measure again.

You should get three different measurements. A barrel pad IMHO adds another varible to the mix and is why I dont use them, period, ever. Just for conversation, W/O barrel pads, I've hung an MTU that was +.100" it's entire length @ 27" and a 1.250" straight for 30" with zero issues.

Each to his own and if it's working for you I say keep it up but, thats not how I'd roll it.

 

[Just Jim]

Re: Question on pad under barrel

Seems as if an easy way to test would be by using a chassis such as the McRees.

Fabricate a floating pad of steel or aluminum, perhaps with a bit of leather or heavy tape on the barrel contact surface, and then drill and tap for a couple of jacking screws to apply upwards pressure onto the pad.

Shoot with no contact, raise the pad to light contact, and then try a few groups with increasing pressure.

Not going to try this with mine though - it shoots too good as is. I'm not changing nuthin'!

Cheers... Jim

 

[19Scout77]

Re: Question on pad under barrel

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: 300sniper</div><div class="ubbcode-body">the only downside i see with bedding a pad in front of the lug is having to redo it at every barrel swap. </div></div>

Interesting point and related topic. I am about to rebarrel a rifle with a pad. Although the barrel is the same contour by the same maker, I assume it will not be a perfect fit in the old bedding. Can I get away with simply relieving the bedding pad? I am going to do this first prior to rebedding, if necessary, but just curious as to others results.

 

[Greg Langelius *]

Re: Question on pad under barrel

The uncertainties I see being discussed seem to be based on a rigid pad. What if that pad was made of flexible material, like RTV silicoe, or a neoprene foam pad? Upward pressure should be constant, or very close to constant, despite heat expansion.

Something to think about, eh?

Then, who cares what the pad is doing when everything is static? My concern is about what happens under the firing cycle. That chamber is going to expand elastically under the pressure cycle. Any rigid contact underneath could very likely generate an upward rebound. If, however, the support was flexible, it could absorb and attenuate the upward force.

Another thing to think about, eh?

Greg

 

[Keith Johns]

Re: Question on pad under barrel

My comp rifle has a pad. Ive gotten the barrel very hot shooting 20+ rounds into one group to check for a poi shift.
No shift.
If someone will pay for the ammo

, I'll fire a 100 shot group as fast as I accurately can. And post target.

Do the M40s get a pad still?

 

[turbo54]

Re: Question on pad under barrel

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: 19Scout77</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: 300sniper</div><div class="ubbcode-body">the only downside i see with bedding a pad in front of the lug is having to redo it at every barrel swap. </div></div>

Interesting point and related topic. I am about to rebarrel a rifle with a pad. Although the barrel is the same contour by the same maker, I assume it will not be a perfect fit in the old bedding. Can I get away with simply relieving the bedding pad? I am going to do this first prior to rebedding, if necessary, but just curious as to others results. </div></div>

You'll need to grind down the existing pad, then redo it...unless the new barrel is within +/- .001" or better in diameter AND concentricity of the old one.

 

[turbo54]

Re: Question on pad under barrel

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Just Jim</div><div class="ubbcode-body">Seems as if an easy way to test would be by using a chassis such as the McRees.

Fabricate a floating pad of steel or aluminum, perhaps with a bit of leather or heavy tape on the barrel contact surface, and then drill and tap for a couple of jacking screws to apply upwards pressure onto the pad.

Shoot with no contact, raise the pad to light contact, and then try a few groups with increasing pressure.

Not going to try this with mine though - it shoots too good as is. I'm not changing nuthin'!

Cheers... Jim </div></div>

Trouble I see with this method is the load.

Seems to me the "ultimate" test would require working up a great load WITH the pad, then removing pad and re-working the load. Then you'd have to replace the pad to prove the test repeats.

 

[Keith Johns]

Re: Question on pad under barrel

Indeed after the pad is created, the barrel cant be turned, set back, nothin. Like you said Turbo it not only has to be the perfect diameter, but after barrel spinning it wont be concentric anymore either.

Maybe there are more reasons not to have a pad than there are to have one

 

[Mgordon]

Re: Question on pad under barrel

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Keith Johns</div><div class="ubbcode-body">Indeed after the pad is created, the barrel cant be turned, set back, nothin. Like you said Turbo it not only has to be the perfect diameter, but after barrel spinning it wont be concentric anymore either.

Maybe there are more reasons not to have a pad than there are to have one </div></div>


I have not found any solid reason to use one yet. I'm still going to play with the idea, but there are just too many variables that can go wrong. I like the idea of more bedding support, but I'm not sure if this is the way to get it.

Mark

 

[2156SMK]

Re: Question on pad under barrel

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: 300sniper</div><div class="ubbcode-body">the only downside i see with bedding a pad in front of the lug is having to redo it at every barrel swap.</div></div>

And this is enough of a reason for me NOT to bed forward of the lug.

I go through a couple barrels a year.
I don't want to re-do bedding a couple times a year.

I have a customer that I put a barrel on his gun every 2-3 months.
It's an unlimited F class gun in a master class unlimited stock.
It shoots itty bitty groups without anything under the barrel and this gun is running straight 1.25" diameter 30" long barrels.

 

[Keith Johns]

Re: Question on pad under barrel

Might see less PCR rifles with barrel pads now.

I may start bedding full length more often, <span style="font-weight: bold">using pipe tape to free float the barrel from the lug forward</span> like Mcmillan and many others do. I think they suggest doing it to their stocks. Seals the for-end and stiffens it all up. Does seem IMO the strongest/best way to bed but it uses up the most epoxy and takes longer.


Thanks for the discussion guys.

 

[RTK]

Re: Question on pad under barrel

I've had a gun full length bedded all the way to the end, it had no adverse heat related POI change, etc. Although I doubt I would do it again.

 

[Keith Johns]

Re: Question on pad under barrel

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: RTK</div><div class="ubbcode-body">I've had a gun full length bedded all the way to the end, it had no adverse heat related POI change, etc. Although I doubt I would do it again. </div></div>

Was it free floated from the lug forward like I mentioned?

Or bedded solid?

 

[RTK]

Re: Question on pad under barrel

No free float, Bedded solid all the way from stem to stern, pillar'd too.
It was in a 300wm with about a #4 contour, The hotter it got the more it liked it.

 

[LRI]

Re: Question on pad under barrel

Having bedded over 1000 rifles in 14 years using a very broad variety of mainstream receiver/barrel/stock combinations I've yet to see a difference.

The only thing it ends up doing is complicating your life once that barrel is shot out and its time to replace it. If the cylinder is bedded, it MUST be done over.

It may be a subject fun to chat/argue/banter over, but in truth there's no one in this industry that I know of who's conclusively shown it makes a difference.

The ONE exception I did see was when building Anschutz 54's using the 690mm length barrel. The guns I built under Neal Johnson's guidance were all done with a pad extending 2.0" datum'd off the front guard screw center.

For whatever reason they shot better this way. 22's are very different from centerfire cartridges. The cartridge basically sucks in its design, it's moving basically 1/3rd the speed of a modern hi po centerfire, and its a cast lead bullet.

All of this boils down to it spending roughly 3x longer in the barrel than a cf bullet. Lots more time for stuff to happen.

I think this is what helped contribute to these particular guns responding well rather than the receiver being too weak to support it adequately.

Think of it this way:

Place your hands together in prayer. Your fingertips are touching one another. Assume this to be the recoil lug of the receiver. Now bend your knuckles so there's a gap between your two hands but keep your palms/fingertips in contact with one another. Now do this back/forth.

You've just simulated what a CF rifle stock does when its set up as a repeater. The thin rib portions along the side of the mag box flex. What this basically means is the stock is acting much like a top fuel dragster. The stock gets shorter. Just like a TF car does when it leaves the line. It hunches its spine so that the rear of the car gains momentum prior to the front of the car tripping the lights. Then it sling shots out of the hole.

Your stock essentially does the same thing. So, if its flexing, just how much support is that little pad doing anyway?

Doubt its flexing? Just look at any large caliber rifle intended for dangerous game. There's a reason why those cross pins/bolts are there. The stock tries to spread and over time it'll cause the wood to split. The forces are real.

If your really, really bored, try this. Get yourself a 4x4 post and cut it in half. Inlet/bed one with the barrel padded and do the other floated. Use the same barreled action.

Then go load up about 500 rounds for each test.

Shoot 100 5 shot groups and get out the calipers. Figure your percentages/averages and report back.

I bet its so minute a difference that it doesn't overshadow the hassles when it comes time to fit a new barrel.

Just my two bits.

C.