Let me preface this entire post with this... My AR shooting has been limited to Army quals almost 20 years ago (expert for what that's worth ) and short range blasting more recently.  I'm new to shooting for groups outside of a 25m zero so go easy.  Honestly , this was the first time I've shot for groups at 100 yards.  I know nothing that follows is impressive.  In fact, I'm pretty embarrassed with it.  Especially embarrassed to post this in the precision rifle forum.  I'm ready to start learning though .

I just had a question.

How much zero shift/groups opening up should one expect while shooting suppressed at a fairly rapid rate?  

My Aero 18 inch fluted SPR barrel shot well enough to satisfy Ballistic Advantages 3 round (yeah yeah ) sub MOA mag fed guarantee...at first.  5 round group was a bit larger.  That was in spite of using American Eagle M193 with rusty me on the an ALG ACT single stage trigger.  I had only put 10 rounds through it to function check it previously, then cleaned it.

Rifle in question.  Budget barrel (Aero fluted SPR), ammo (AE M193), scope (PA GLx 2.5-10x44), single stage trigger (ALG ACT), and out of practice shooter behind it...lol.



Zeroing at 100 yards with American Eagle M193 cause that's what I have on hand.  Want to try some match stuff soon.




Anyway, after about 100 rounds though it without letting it cool, the grouping had opened up and shifted quite a bit.  There was considerable mirage coming off the barrel and suppressor.  I was pretty much just loading mags and letting it rip.

Is this kind of group shift and growth pretty normal with a hot suppressed gun and bulk ammo or was I just getting way sloppy?




Banging a steel silhouette at 300 was still stupid easy the whole session though.  Was double tapping it for fun getting the 2nd round off before I even heard the first hit without even having to try because I was getting bored.






All in all, it was a fun day.  Just not great to end with what seemed like a shifted zero and much larger groups.  Stayed minute of torso silhouette the whole time though at least

Some barrels will do that

Maybe fatigue was setting in? I know I get sloppy pretty quick.

That's what I was wondering about.  Didn't really know what to expect.






That is certainly a possibility.  

I was walking back and forth some between shooting to put targets up, then getting right back on the gun and banging away.  

The mirage off the suppressor was really messing with me too.  Everything was blurry and moving around.  I felt like I was on target when I fired though.  It looked like my shots should have been where I wanted them when they broke, then I'd see the hole way off from where I would have called it.  Didn't feel like I was flinching or pulling shots.  It was pretty frustrating.

Probably took the suppressor 30+ minutes to cool off when I was done.

It really depends on the build. Some rifles with tight tolerances won’t shift poi with heat. Others might if tolerances are loose. After 100 rounds yours looks like it is shifting.

Mirage is what usually costs me group size but not group shift.

Your group with xm193 is actually pretty good. I would have expected bigger.

Enjoy!

Definitely not a tight tolerance build lol.  

I put it together in a barn on a wobbly vice with an Aero receiver set and barrel.  The nub on the Geissele barrel nut wrench broke off (probably due to operator error with the torque wrench) so I just called it good since the barrel nut hole was slightly egged.  I'm guessing I went a little over 40 ft lbs....  The wrench was set at 40 but I forgot to turn the knob to lock it at the bottom.  I was wondering why it wouldn't click.  Oops.  Lesson learned .

I'll have to get it out again soon and see where it's at cold.



Thanks!

You’ve already gotten some good responses, but still. It’ll happen. Heat will make things expand, and with different metals in contact they’ll expand at different rates, etc. So any slop between mating surfaces, that expansion will find that slop. When we are talking about fitments like barrel extensions against barrels, that’ll absolutely cause groups to start walking and widening. Tight fitments can help, but help is the key word. Some barrels are better about it than others, but better is the key word.

I guess the takeaway is that while it can be mitigated, it’ll pretty much always happen when measured with a fine enough instrument. Hell I guess it is happening with every shot to a degree, not that it happens in much of a meaningful way at such low temperature differences.

I’ve been getting several precision rifles up and running this summer, and it’s been HOT out. When tweaking things, that POI shift and group expansion can overshadow whatever accuracy tweak I’m trying to measure. I spend a lot more time letting my rifles cool than I do shooting them under those circumstances.

Thanks for the detailed response.

That all makes sense.  I was expecting the groups to open up a little, but that POI shift was my main concern.  

I'll have to be more patient and let the gun stay a little cooler next time.

I would highly recommend a suppressor cover. For precision purposes (ie not burning it down with mag dumps), even the Mechanix cover has done fine for me.

I'm definitely planning on getting one now!

As stated, it can and does happen.

Mirage will also fuck groups up too simply due to the visual distortions when looking through the scope.

Especially this time of year, absolutely.