first round cold bore poi shift

[ajridgedell]

What can cause first round or cold bore poi shift in a suppressor. My blk has repeatable 3-4moa vertical shift with cold bore, first round is high. Only the first shot, all others group about 1moa. After about 10 minutes of rest returns to first shot hits high. These are hand loaded sub sonic rounds. 2 Form 1 suppressors have a similar shift. The smaller suppressor only moves about 2moa. I am wondering if there is a known characteristic of the suppressor that affects first round poi. Thanks for the help.

[sniper1]

This is evident in all rifles with or without a silencer. Head over to snipershide.com for massive amounts of info on the subject.

However the short of it is:

yes it is repeatable, you should log the shift so you know what to expect with the first round.
usually caused by changes in barrel temps, however some claim it has to do with burnt powder in the barrel...YMMV

my educated guess is that as your barrel warms it flexes differently. The harmonics change when you add a silencer. In theory the heavier can will cause a more dramatic change in harmonics, hence, the larger shift.


I am sure someone smarter than me will come along with more info.

[continuity]

... I am sure someone smarter than me will come along with more info.

Have no idea if your screen name implies RL experience, but will opine that your post is full of win. Absolutely agree with everything you posted.

[doubloon]

Agree with above, the only other explanation I've seen anyone dump any energy into is clean bore.

http://www.youtube.com/watch?v=y-ukwrmca9s

I tend to believe a quality thick barrel shifts less

Here is one man's technique for cold bore mapping.

http://www.youtube.com/watch?v=NeGseLFpHks

[Historian]

This is evident in all rifles with or without a silencer. Head over to snipershide.com for massive amounts of info on the subject.

However the short of it is:

yes it is repeatable, you should log the shift so you know what to expect with the first round.
usually caused by changes in barrel temps, however some claim it has to do with burnt powder in the barrel...YMMV

my educated guess is that as your barrel warms it flexes differently. The harmonics change when you add a silencer. In theory the heavier can will cause a more dramatic change in harmonics, hence, the larger shift.


I am sure someone smarter than me will come along with more info.

+1!

And you have demonstrated being darned smart.

A few third order effects could also come into play:
shooter's first flinch, grip/slip, breathing.

As one 'ages' like fine French wines the tertiary effects get magnified ... and
one's denial mechanisms like blaming the influence of the "earth's rotation, continental drift, etc.
affected my shot" become too tiresome to invoke.

Has anyone given their piece first to another shooter to fire a few rounds and then
seen if POI shifts still occurs with one's first shot. When this test was done
so many years ago I was taken aback to see that my first shot still had a first shift.

Definitely it was the passing of that SR-71 circling overhead taking pictures of us.
Certainly not the unrealistically fantastic shooting by that young Mustang Marine Major Jouster
who out shined us.

[ajridgedell]

Thanks for the advice, for some reason I was totally convinced it was a design flaw in my suppressor. took it off today and shot and the poi shifts in a similar pattern. No other rifle I have does this and precision rifles is my hobby. I'll try some things with the rifle I built it. Noveske barrel 10.2 inches shouldn't do this I wouldn't think, I'll move to another forum to pursue rifle issues. Thanks guys

[ToyotaGuru]

My 300blk does the same thing, 9.2in AAc barrel. Had multiple people try shooting it, changed loads, bullets,, can/no can, oal, etc and was always the same on the first cold shot. I found that going to the max crimp with a Lee factory crimp die made it a lot less, now down to .5-.75 inch high on the first shot.

[ajridgedell]

I know I'm getting away from "silencer talk" but I wanted give an update to what I have found. The syrac ordnance gas block raises the gas tube high enough to touch the yhm free float tube. I am theorizing that after one shot the gas tube heated up enough to change the harmonics or simply push down on the barrel. Beat the gas tube down some and can't wait to try it out tomorrow.

[Historian]

I know I'm getting away from "silencer talk" but I wanted give an update to what I have found. The syrac ordnance gas block raises the gas tube high enough to touch the yhm free float tube. I am theorizing that after one shot the gas tube heated up enough to change the harmonics or simply push down on the barrel. Beat the gas tube down some and can't wait to try it out tomorrow.

Please give the range you were shooting at , the actual amount of shift on the target, quadrant,
and if you were bench resting. This would be valuable information to accumulate.

Thanks.

[ajridgedell]

80 yards, only the first round would hit 3 inches high. Shot again today after fixing the gas tube and problem solved. cold bore shot and consecutive shots all group in 1 inch. Hopefully this can help someone else too.

[SRM]

I use the 'clean bore' with my rifles..

After the fouler, they hit the same place so I leave en 'dirty' for a while.

[Pman5KMO]

I have only had that much shift after doing a thorough cleaning of the barrel on my rifles. I'm talking no trace of any fouling of any kind. My rifles have negligible shift from the first round till mirage once you foul the bore with a single round... Which is why I only do a thorough barrel cleaning immediately before heading to the range and generally stopshort of thorough any other time.

It has much more to do with barrel quality and construction than a suppressor. Some cans will amplify this effec. due to harmonics.

Watch high speed video of a rifle firing and you can see thinner barrels flopping around like a wet noodle. This is also why my rifles have good high end barrels.... IOW this is the primary difference between a $200-300 barrel and a $400+ barrel.

Other factors include mount/muzzle device type/design, crown, and bedding/stock or receiver interface design.

A good high quality precision rifle will have much less shift than your typical off the rack rifle. This is why some precision rifle builds cost $3k+ and why people pay bog bucks for lug lapping, truing/blueprinting, hand lapped bores, etc. And even then a .5 shift is considered really good.

If this only happens with a squeeky clean barrel stop cleaning it so much. A minimally fouled barrel tends to shoot better than a squeeky clean barrel for that very reason. I boresnake my barrels for most cleaning with some patches run through from time to time but only when I gauge the throat do I get it squeeky clean... That or the bi-annual deep cleaning

[SRM]

I have only had that much shift after doing a thorough cleaning of the barrel on my rifles. I'm talking no trace of any fouling of any kind. My rifles have negligible shift from the first round till mirage once you foul the bore with a single round... Which is why I only do a thorough barrel cleaning immediately before heading to the range and generally stopshort of thorough any other time.

It has much more to do with barrel quality and construction than a suppressor. Some cans will amplify this effec. due to harmonics.

Watch high speed video of a rifle firing and you can see thinner barrels flopping around like a wet noodle. This is also why my rifles have good high end barrels.... IOW this is the primary difference between a $200-300 barrel and a $400+ barrel.

Other factors include mount/muzzle device type/design, crown, and bedding/stock or receiver interface design.

A good high quality precision rifle will have much less shift than your typical off the rack rifle. This is why some precision rifle builds cost $3k+ and why people pay bog bucks for lug lapping, truing/blueprinting, hand lapped bores, etc. And even then a .5 shift is considered really good.

If this only happens with a squeeky clean barrel stop cleaning it so much. A minimally fouled barrel tends to shoot better than a squeeky clean barrel for that very reason. I boresnake my barrels for most cleaning with some patches run through from time to time but only when I gauge the throat do I get it squeeky clean... That or the bi-annual deep cleaning

what he said, thank you. the bore snake is your friend!