Free floated barrel = POI shift?

[17K]

I was zeroing a 16" BCM that I recently swapped the 13" KMR for a 15", following instructions to the letter.

I got it zeroed on the bench, riding sandbags. Shot a few groups and was satisfied it was printing where I wanted so I left the bench and went to the bipod. .5 mil shift down from on the bench. Went back to the sandbags, POA=POI. Back to the bipod, .5 mil low.

WTH??

[gunnerblue]

Flex in that long hand guard coming into contact with the gas block? Perhaps you’re giving more downward pressure leaning into the bipod as opposed to sitting on bags.

[opngrnd]

I was zeroing a 16" BCM that I recently swapped the 13" KMR for a 15", following instructions to the letter.

I got it zeroed on the bench, riding sandbags. Shot a few groups and was satisfied it was printing where I wanted so I left the bench and went to the bipod. .5 mil shift down from on the bench. Went back to the sandbags, POA=POI. Back to the bipod, .5 mil low.

WTH??

Sounds like the shift is repeatable. Are you getting on the rifle the same way each time?

[user]

Cold bore only or entire group 1/2mil low?

[markm]

What sight system?

[17K]

Sight system is a Nightforce NXS 2.5-10X42 in a Nightforce Uni-Mount. Not bridging the gap.

Entire group, multiple 10 round groups with the same ammo just to make sure it was really a .5 mil difference.

I know I'm loading it different on the bipod vs the bench, but that's the whole point of free floating the barrel, no?

No contact with the gas block. The reason I swapped the 13 for a 15" was because my Keymod backer nut for the picatinny section was touching the gas block when I loaded the bipod. That caused noticeable stringing issues that the longer hand guard cured, but the POI shift between positions has me a little irritated.

I haven't shot an AR off a bench in a couple of years, so this may be a common thing, I just didn't realize it. I thought the free floated barrel would be like on a bolt gun. Do whatever you want, it stays the same.

[Matt in TN]

A repeatable shift like that sounds more like it's caused by your position than your hardware. In Highpower Rifle competition it's a known fact that you have different zeros in different shooting positions - but to be fair this was with iron sights (and a free floated handguard). I'd have to look back at my logbook, but I don't think it was ever as much as 2 MOA/0.5 mil.

You don't normally see it as much with a scope that's properly adjusted for parallax, but I can see how it would shift when you go from shooting from bags to a bipod on a hard slick surface (guessing here). Learning to preload a bipod and minimize bipod hop is a real skill.

All that to say: "don't worry about it". Quit shooting off the bench anyway. Know your zero from the main position from which you shoot for accuracy, and then practice various field positions and know how those postions affect your POI.

[grizzman]

If you're heavily loading the front of the handguard with the bipod vs almost no load at the middle/rear of the handguard at the bench, then differing amounts of pressure is being applied to the barrel nut....which is attached to the barrel. I'm surprised that the shift is .5 mil.

The only way I'd expect absolutely no change in POI is if the handguard were attached to the upper receiver.

Also, loading the bipod is likely reducing some of the muzzle rise, which will lower the point of impact.

I'm going to test this myself during my next range visit.

[grizzlyblake]

Do you have a sling that you can "wrap up" with to see if it changes POI as well? That's certainly an interesting deal as I had always assumed the free floated handguard would allow you to do anything regarding pressure/position and not impact the POI vs POA.

Is this same thing seen on the DD type uppers where the handguard is attached directly to the receiver, not the barrel nut?

[LMT/556]

I'm noticing the same thing switching from a sandbagged rest to a bipod prone with similar hardware, and attribute it entirely to the way I interface the rifles differently.