Point of Impact Shift Mystery

[exd3686]

I went shooting today and had a very strange POI shift occur. First, the details: I recently had my Jerod Joplin PAI Rem. 700 .308 removed from the McMillan A5 and installed in a AIAX chassis. The lug was a little long and was bottoming out in the lug pocket on the chassis. I had my local gunsmith mill 50 thousands of an inch from the lug and now fits perfectly in the stock. My reloads were from a single batch of Winchester brass, CCI BR2 primers, Sierra 175gr MK HPBT, with 45.3 grains of Varget traveling 2693 fps through my 22" barrel and suppressor.

I zero'd at 100 yards un-suppressed with a sub-minute dead center group. Then zero'd suppressed at 100 yards with NO POI change with a sub-mintue dead center group. Went back to 600 yards with suppressor and consistently hit a 9x15" steel plate and some moving/blowing helium balloons. Took a 30 minute break and then went back to shooting. Couldn't hit a darn thing. My buddy tried as well and could hit a thing. His GAP continued to hit targets without elevation or windage changes. I changed my elevation settings from 4.1 mil to 3.5 mil and walked in the shots to the steel plate....got very close to hitting the plate back at 4.1 mils.....? Went to 100 yards and shot a 3 shot group suppressed and got a sub-minute group but now 0.8 mils high (didn't chromo but ammo from same batch). Dialed 0.8 mils and hit dead center with a 1/4" two shot group.

- Scope is mounted solid
- No baffle strike marks on suppressor
- Same reloading batch using Forster competition dies, RCBS Rock Chucker, and RCBS ChargeMaster

Any idea on why the POI changed 0.8 mils within 30 mins. with everything being the same????? I'm stumped!

Thanks,
exd3686

 

[dtibbals]

I'm the friend with the GAP...I guess it is time to tell him during the break I took his scope and turned it around on him...LOL just kidding. After the break we switched rifles and I couldn't hit the target at all! I have shot this rifle in the past and never an issue...frankly so accurate it gets a little boring. So I was happy to get back behind my rifle and hit, hit, hit, hit no issues at all. He did as he mentioned above I was spotting for him and we where stumped! So going to 100 and letting the rifle cool off a bit we got the results above. I have never seen a rifle do that let alone one of that quality. Hope you guys have the answer!

 

[ogonzalez13]

How hot was it there today. Was there a temperature increase in the 30 mins?

 

[exd3686]

Zero'd at 74.1 degrees with 70% RH and 76.7 degrees and 64% humidity when shooting. My buddy was shooting along side me and had no POI shift. Still a mystery!

 

[Barney88PDC]

Bed the chassis. Yes I know defeats the point of the chassis. But bed the chassis.

 

[maggitas]

Did you leave the cartridge box open. Maybe the sunlight heated up the ammo.

Johnny

 

[exd3686]

Barney88PDC -

My McM A5 was fully bedded but most of what I read stated that it is not necessary to bed the AIAX chassis. I have read that some will bed the lug pocket on the chassis. Is this what you're referring to when bedding the chassis. Does a non-bedded chassis cause these inconsistent POI changes?? Thanks.

 

[exd3686]

Johnny -

I did leave the box open while firing at 600 yards....but let them cool before heading back to 100 yard for zeroing and still getting the 0.8 mil increase.

 

[dtibbals]

I have the same set up but built by GAP and in a different caliber. My rifle was doing fine but GAP did bead the lug but that is it. Here is a pic of the two side by side...his is OD and mine is Patriot Brown.

 

[Arbiter]

Possibly a dumb question depending on your cleaning regimen, but how long since you'd last done a thorough cleaning of the barrel? Wondering if copper build-up could have built to the point of influencing the POI...

 

[maggitas]

EdX,

You have to ask yourself what changed. Here are some of my suggestions.

1. New Stock. I assume this did not happen with your McMillan Stock so I would check or recheck the Stock placement.
2. Without seeing your rig. Clean muzzle adapter, bolt check bolt for copper fouling debris.
3. Adjustable stock settings, check they have not changed. Also double check scope settings.
4. Ammo: If you got a chrony I would check to see that it is consistent.

 

[maggitas]

EdX,

You have to ask yourself what changed. Here are some of my suggestions.

1. New Stock. I assume this did not happen with your McMillan Stock so I would check or recheck the Stock placement.
2. Without seeing your rig. Clean muzzle adapter, bolt check bolt for copper fouling debris.
3. Adjustable stock settings, check they have not changed. Also double check scope settings.
4. Ammo: If you got a chrony I would check to see that it is consistent.

 

[oldfatguy]

The smith milled 0.050" off of the lug. Is there any clearance between the lug and the bottom of the pocket, or is it line to line?

OFG

 

[Barney88PDC]

I had a rifle with basically a chassis system, aluminum block, in it. I had a first round POI change of 0.2 High and 0.1 left. Rest of the mag would stack rounds dead center. Let the gun cool and same thing. I did a full bed job and the problem went away. I have a buddy who swapped from an A5 to an AX. He did a full bed job. Next F Class match he won with his best score ever. Bed the chassis.

 

[trilogymac]

I had a R700 .223 built with an AICS and requested it not be bedded to see for myself if an unbedded chassis was an issue. The smith told me that it was an issue on some builds and some worked out well. I had flyers and wandering zero. Not bad, but enough to rethink the unbedded chassis. Had him drop the BA in an HS 5R stock and properly bed it. Issues went away.
If I had bedded the AICS I'm sure I would have seen the same results. The AICS just feels like a boat anchor to me.

YMMV.

Hope this helps.
Trilogymac