POI shift through over-caliber suppressors

[TehLlama]

I'm looking at picking up a brake attached Ti suppressor (in 30 cal) with the intention of moving that can between a 260 rifle (KMW-LRS 260 or an LT PredatAR rebarreled in 260) and an eventual 300BLK SBR.

For removal/reinstallation on the precision rifle, is POI shift something I need to be really concerned about using the manufacturer muzzle brake/thread on attachment?
Should I be looking at the more expensive of the two units based on the POI shift?

Right now I'm comparing between the more affordable SAS-LLC Arbiter and the TBAC 30-BA

[orkan]

POI shift is a product of one thing:

Barrel harmonics being changed by a weight being hung off the end of the barrel.

You could argue a secondary effect caused by harmonic resonance due to the hollow chambers... but you'd need a lab to test it and wouldn't be applicable in the real world.

This is clearly illustrated by the fact that lighter suppressors exhibit less POI shift than heavy ones, as a rule.

Proof? Take a straight barrel with no taper, at about 18" in length and hang a 2lb suppressor off the end. It will likely not shift POI at all. Now hang the same suppressor off a 24" sporter barrel.

However this is not a constant, as all barrels will resonate slightly different depending on the barrel and the load used. This is the same principle we use to tune loads to our barrels, and is also why I do load development with the suppressor on, if the rifle is primarily to be shot that way.

There is a bit more to it, but that is the nuts and bolts of it. POI shift with any suppressor and any rifle, will generally be consistent and repeatable... thus - easy to account for.

[Anchor Zero Six]

POI shift when changing harmonics is a given as orkan stated.

Whats important is that its a consistant shift meaning repeatable and replicatable.

Both of the cans you are looking at have good reputations so I would say you would be GTG either way.

You should keep data on the zero for both supressed and unsurpressed. An example of application would be that you zero the rifle unsurpressed first. Attach the can rezero and log how many mils of correction were dialed. Do this several times and average the correction.

I would also advise to do this over several firring sessions at varying temps and altitudes. If you have enough data in your log books you can predict your cold bore shots with a surprising degree of accuracy. Just takes a while to get the data but who cares its an excuse to go shoot right?

[orkan]

If you have enough data in your log books you can predict your cold bore shots with a surprising degree of accuracy.

I've yet to see any change in POI from cold bore, to 5th shot in any of my rifles. Have you proven cold bore differs for yourself, or have you just read about it?

I'm convinced its a matter of cold shooter much more so than cold rifle. I've not had anyone show proof of otherwise.

Want a test? Take two similar rifles that you know to exhibit excellent accuracy. Shoot one for 20rnds or so until you are warmed up and comfortable. Then swap the rifles. See if your POI is really any different with the other rifle. In my experience, this proves cold bore to be a myth. Granted, not many people have two rifles with identical stocks and triggers to replicate the same feel... but some do. I've done this test on several people, and it always ends the same. Cold bore doesn't exist... but cold shooter does.

One of the best ways to overcome it, is to get in position, and dry fire about 5-10 times on target until you are comfortable in the fundamentals and verify that you have built a solid position. This is especially helpful when shooting heavy recoiling rifles.

Now, there is one factor that is often overlooked. Shooting sessions never have the same environmental conditions as the one previous. So another thing that people attribute to POI shift is actually changing environmental's which affect their zero, yet were unaccounted for during the shot. Most people don't shoot enough to truly track this phenomenon. Want another test? Shoot your rifle at 300yds, 5 shots, every single day... for a month. Record all data... and compile it for eval at the end of the month. You really want to learn something about yourself and your rifle, this is one very efficient way to do it.

Driving a rifle properly, like any motion where precise repetition is key, is always done best after a bit of practice during each session. This is a fact. The only way to get rid of it, is to train it away through repetition.

[Anchor Zero Six]

Some good points there and I agree with them. I think my wording was poor or rather upon consideration the term itself is missleading. I have never thought about the literal definition of the term cold bore and always took it to mean the first shot fired.

I think you nailed it by saying cold shooter and enviormental conditions. What I was getting at is knowing what corrections need to be made to get a first round hit at the start of a new day of shooting (possibly at a different location).

And no I have not seen evidence of "cold bore" shots differing from others throughout the day in regards to accuracy (moa) but I have seen variations in point of impact vs point of aim. POI vs POA obviously attributed to changes in enviorment. This is assuming a normal rate of fire as I think all have seen hot barrels walk.

On of my prefered shooting drills is the Dot drill and the very first round fired is a "cherry popper" for the reason you mention its gets you past the cold shooter stage.

[markm]

I've yet to see any change in POI from cold bore, to 5th shot in any of my rifles.

What about CLEAN bore? We cleaned bores mid day in a precision class I took years back. .(223 Rem 700 with a Shilen bbl) And it took 3 rounds for the POI to get back home.

Now My Rem 700 will shoot clean cold bore absolutely perfectly. I mean it's first shot is sometimes the best shot of the day.

[rickp]

What about CLEAN bore?

IMO, this a whole new discussion. IMO, cleaning midway through a course of fire is asking for misses or poor accuracy until the bore is fouled up and the natural imperfections are filled smoothed over with carbon and copper.

[orkan]

IMO, this a whole new discussion. IMO, cleaning midway through a course of fire is asking for misses or poor accuracy until the bore is fouled up and the natural imperfections are filled smoothed over with carbon and copper.

This.

My rifles are always in a "fouled" condition. My custom barrels take as little as 3 rounds to come in, after being cleaned... while most of my factory barrels will take from 10-20.

[rickp]

This.

My rifles are always in a "fouled" condition. My custom barrels take as little as 3 rounds to come in, after being cleaned... while most of my factory barrels will take from 10-20.

Same reason mine is fouled up too. We're on the same page!!!!

[TehLlama]

I've drawn the ire of plenty of my armorers by refusing to clean the barrel during our qual week, and my old issue rifle was a real tack driver if I kept the bore flithy.

I'm guessing ambient temperature will probably have some effect on zero, but I doubt it'll be terrible pronounced between suppressor on and off.

To a degree I feel like the AAC MITER multi-ratchet system was gimmicky from the start (shouldn't all cans be radially symmetrical unless they're really half-ass?), but the brake with thread-over should give me fairly consistent results, so I'll work with that.

I'm also assuming that going Ti will also help with this.



As far as figuring out the various shifts across conditions and loads, I'm working on a pretty detailed statistical DOE setup that I think can minimize cost and improve load development as well as give residual information about POI shift and temperature effects on loads.