Is my rail causing shift

[aslrookie]

At yesterday’s match, from positions that required me to utilize my GC bag instead of the bipod, my impacts were consistently low. At first I thought something was wrong with my DOPE because I adjusted up 5 tenths and got impacts from the rooftop stage.

I knew my MV wasn’t under 2600fps with my 224V with a 24” barrel, and I confirmed my data the day prior out to 765. I trued my MV based on impacts to 2662fps through AB app. A shooter let me borrow his magnetospeed just to see, and my MV was 2661fps.

I’m thinking it’s the rail, barrel nut interface causing the shift. I was aiming shoulder height on a full IPSC plate at 508 for a PRS skills stage and was still connecting at the bottom.

From prone on the bipod, my DOPE was spot on. My rail is an SLR rifleworks solo lite rail. I am considering throwing this rail away or just going to a monolithic upper.

Thoughts?

 

[aslrookie]

Another thought, with a 24” barrel and a suppressor could it be applying torque to the upper receiver when the rifle is supported at the receiver instead of by the gas block with the bipod?

 

[Rudy Gonsior]

Switching from bipods to a GC bag could definitely be an issue, especially with lighter weight rails/barrel nuts. Shooting off of tripods I’ve seen a students DD rail cause 3-4 MOA of shift (low), dropped her back to prone and she was right back on. My small frame AR with Geissele Mk8, which is has a fairly solid barrel nut, only exhibits a 1 MOA low shift from the tripod. My OBR on the other hand has zero shift from prone to tripod, of course there’s a lot more “meat” on the OBR and it shows.

Switching to a monolithic style upper could probably fix your issue, the Geissele rails is another option (though not perfect) I’ve also had really good luck with the Seekin rails.

 

[Back Spin]

To the OP, have you ever done any side by side groups at 100 from prone bipod and GC bag to see if there is any offset to account for?

When prepping for my first PRS match recently (with a 224V semi) offsets from different positions were something I paid attention to. What I found was a barricade bag was about .2-.3 mil low compared to prone.

When I added a bungee sling to aid in support I saw the elevation offset disappear, but a lateral shift occurred if I cinched too tight on the sling.

When zeroing, I used a light hold/load. How you zeroed the gun could affect the offset.

 

[bfoosh006]

To the OP, have you ever done any side by side groups at 100 from prone bipod and GC bag to see if there is any offset to account for?

This... before you "pitch" the handguard I'd do some tests.

It sounds to me... you may be right.

 

[aslrookie]

I haven’t done any testing to see the shift. This is the first time I’ve encountered this. I’ve always shot precision work with a shorter barreled AR10, bolt guns, and my 6.5G that had a shorter/lighter barrel with a reflex suppressor so the weight wasn’t so far away from the barrel extension.

During the match, it was consistently .5mil low when not using the bipod.

I still have this Howa 1500 HCR that hasn’t been used, so I am switching to that for my precision work now. A 12.5” 6.5G SBR sounds like a great replacement for the 224V.

I played around with the idea of just getting a new upper that’s known to be better for supporting heavy barrels like the VLTOR MUR, but with this Howa just sitting in the safe...might as well use that instead.

I was just curious if this was a real thing or if I was just consistently pulling shots .5mil low which seemed unlikely.

 

[Yondering]

Switching from bipods to a GC bag could definitely be an issue, especially with lighter weight rails/barrel nuts. Shooting off of tripods I’ve seen a students DD rail cause 3-4 MOA of shift (low), dropped her back to prone and she was right back on. My small frame AR with Geissele Mk8, which is has a fairly solid barrel nut, only exhibits a 1 MOA low shift from the tripod. My OBR on the other hand has zero shift from prone to tripod, of course there’s a lot more “meat” on the OBR and it shows.

Switching to a monolithic style upper could probably fix your issue, the Geissele rails is another option (though not perfect) I’ve also had really good luck with the Seekin rails.

It's not the rail or barrel nut that causes what you're seeing, it's flex in the upper receiver. It doesn't matter how much the rail itself flexes, as long as the optic isn't mounted to it the same amount of load is applied to the upper. It's flex in the interface between scope mounting and barrel mounting that causes POI shift - that interface is the receiver itself.

A monolithic upper can definitely be stiffer and improve things there, as you said.