SOLVED: First shot of each group low and right

[Smilner01]

I just built a Grendel and have only put two boxes of Hornady 123 SST through it, so still breaking in. 16” AA barrel, Slr adj gas block, suppressed. My first shot from each 3 shot group is 3” low and 1/4” right at 200yds. I would understand to a point if this was on the first 3 shot group cold bore, but So far I have shot 3 rounds, loaded 3, shot 3, loaded 3, etc. not giving the suppressor or barrel long enough to return to “cold” in between and each first round is low and right (other two rounds are within 1/4” or so of POA). Had a friend shoot a couple groups as well to rule me out. Any ideas other than more break in?

[Bigs28]

Did you try shooting without the suppressor?

[Smilner01]

Have not yet, going to give that a shot at my next range trip. I have had POI shift suppressed vs unsuppressed on other rifles, but not a consistent flier. My first range trip was short due to a lack of available ammo, wIting for Hornady to catch up. Another thing I was going to try was just loading up a mag and ruling out a variance in first round manual feeding.

[VASCAR2]

This might sound crazy but if the bullets are getting damaged or pushed deeper in the feeding cycle it could affect the eventual point of impact. Some mags don’t let the nose of the bullet rise enough to smoothly hit the barrel extension. Some times the ejector doesn’t allow the cartridge head to slip under the extractor claw on feeding.


I suggest you remove the sharp edge on the ejector as it will not hurt anything and can improve feeding. I’d also measure the cartridge overall length and feed a round into the chamber from the mag. Eject the unfired round and measure the cartridge overall length.

Curious if the bullet was seated any deeper from feeding from the mag. If the round wasn’t seated deeper then feed a round from the mag and shoot. Pull the next unfired round out of the chamber and measure cartridge overall length. If the cartridge is seated deeper you might have found part of your issue. If the rounds are not being pushed deeper at least you’ve eliminated one possibility.

I’d also inspect your barrel for a clearence issue. During recoil the barrel/gas block can contact the hand guard or there might be a clearance issue with the supressor. Normally an issue (fliers) like this is found by eliminating possible culprits.

Did you use a lapping tool on the front of the reciever and bed the barrel in the upper reciever? How was the fit between the barrel and upper. If you didn’t bed the barrel in a loose fit barrel to the reciever this could be a contributing factor.

[Smilner01]

Good idea about the OAL, I’ll check that out when I get home this evening.

ETA: OAL is the same on all three...round out of box, round fed from manual release of bolt, and round fed after shot round.

All I had a chance to check yesterday.

[NugginFutz]

Here's a simple test. Load each round the same way. If it is to allow the bolt to strip off the round when letting the charging handle go, make sure you do that for each of the three. Load the magazine with one round, only. However your get the first round into the chamber, do the same for each subsequent round.

If the POI still shifts, it likely is not a feeding / seating issue.

[1Shot]

It could be the ammo. Shoot more than three round groups and see what happens. I have two Grendels and one HATES 123 SST ammo averages 3" at 100 and will not shoot any hand load with the 123 SST bullet accurately and the other rifle will only do about 1 1/2" 5 shot groups with it. Both rifles do MOA or better with hand loads. The Hornady 123 SST is the only factory ammo I have shot in them.

[Smilner01]

Well, the culprit was the first round loaded using the bolt release. When I charge the first round by pulling back the charging handle and releasing it all goes well. Charge the first round with the bolt release and the shot is low. Not sure it makes good sense to me, but several more boxes though and it holds true.

[Bigs28]

Did you pull a bullet after sending it with the bolt release to see if it's damaged in anyway.

[Smilner01]

Yes, when I was measuring OAL to see if there was any variance after charging with bolt release vs charging handle vs cycling after shot fired. No indication of damage.

[LRRPF52]

Well, the culprit was the first round loaded using the bolt release. When I charge the first round by pulling back the charging handle and releasing it all goes well. Charge the first round with the bolt release and the shot is low. Not sure it makes good sense to me, but several more boxes though and it holds true.

I was going to recommend sling-shotting it versus bolt hold open and release on a fresh mag.

The AR15 is meant to be chambered with a rapid cycling of the charge handle from a bolt closed position, as this mimics the actual cyclic as best as we can when loading, though nowhere near as fast.

The exception is once you run dry and do a reload, but you can still cycle the charge handle for that action, even though it's meant to be returned to battery quickly with the bolt release paddle.