Today I went to the public 100 yard range.
Nice concrete shooting benches, little or no wind (1 MPH according to weather.com) 60 degrees and sunny.
Mounted a 4-12x scope on my DD 18" S2W 1:7 CL CMV CHF barrel with Osprey gas system upper (not free floated).
Windham Weaponry lower.
Hung it on an el cheapo Sportsman's Guide bench rest.
Took 30 rounds of Wolf WPA to get it on paper (typical $hit, couldn't even tell where the rounds were hitting for the first 20 shots...)

Then I turned my attention to a fresh target with 10 rounds of Federal XM193F.
3.3 inch group.
Didn't really notice at this point the barrel is HOT.
Another target, 10 rounds of Hornady TAP 75 grain (the best commercial ammo I could find, still looking for the Black Hills 77 grain).
2.3 inches, WTF?

Moved the WW upper home to its lower, now shooting the WW HBC as purchased (except for the 4-12x scope).
CMV CL 16" 1:9 twist.
20 rounds of Wolf WPA to get it on paper.

Then I noticed the guy 2 benches down spitting cases at my feet, funny looking AR brass with a big hole at the business end.
Went to investigate.
Ran a few rounds through his 6.8 SPC, BS'ed for a little bit.

Back to the WW, which has cooled a bunch, although the sun didn't let it cool down fully... phosphated barrels absorb a lot of heat from the sun. Still, much cooler than the DD was, no doubt.
10 rounds of XM193F and a fresh target, 2.09 inch group. Not bad for ball ammo, I suspect.

Then I loaded up 10 rounds of the TAP. Now when I say loaded up, I mean 3 rounds each in 3 mags plus 1 round in one mag. State law at the public range.

Start with the single round.
Shoot.
Looked for the hole with 60x spotting scope. Nothing.
Inserted a 3 round mag. 3 more shots disappear without a trace.
No holes through the backing or the wooden frame.
At $1/round, I decided not to shoot anymore of the TAP with the WW on this day.
I thought the 1:9 twist would be okay with 75 grain, apparently not.
Or maybe something else is going on, maybe the cheap a$$ scope I was using broke.. but I didn't think to test it with Wolf to see if it lost it's zero, as I had to split.
In retrospect I should have taken 5 minutes to verify proper function.

I know my methodology is not perfect, but I really expected the DD barrel to do better.

Thoughts?

Joe

A hot barrel will affect point of impact more than anything else. Your POI will also change a lot depending on different loads/ammo. Your "cheap a$$ scope" is probably the biggest problem. What scope/mount were you using?

Most people recommend 1:7 if your goal is to use heavy-for-caliber bullets.

I experience the POI shift going from Wolf to american ammo, but the TAP and Federal groups were centered 3/4" apart at most.

What I've read indicates that a 75 grain bullet is the heaviest (actually the longest) I should expect some measure of accuracy out of a 1:9 twist rifling, so I did not expect the bullets to disappear entirely.
It is possible that the scope bit the dust but the timing is impeccable... for instance, I get lackluster results from the DD 1:7 and then move it to the 16" 1:9 and get 2 MOA with ball XM193, then the scope breaks after the 10th shot of XM193 , before the first round of TAP?

I should have bought a lottery ticket with those odds.

The scope came with 30mm rings. I bought it from Sportsman's Guide after reading a page of excellent reviews, including several AR owners.
scope
I won't know until next range trip whether it expired, though.

Joe

First of all, scrap that piece of CRAP scope! You aren't doing yourself, or your rifle, any favors by using it. Your firearm is only going to be as good as the optics that you put on it, and those cheap Chinese optics are shit right out of the box. The 'good reviews' that they get are from guys who have no idea what a quality scope is. Second, if you want to check the accuracy of your rifle (once you top it off with a QUALITY optic), stick with one type of ammo. Once you're satisified with your 100 yd groups with that ammo, then move on to different ammo and see where it prints in comparison to the ammo that you used to zero, then make the necessary windage/elevation adjustments. All ammo is going to print differently, even the same ammo from lot # to lot #, but not as much as a totally different brand, weight, etc.

As far as barrel temp being a prob you should read the  1 shot group post by malon.

My Aim sport recon 1.5-4x30mm is an amazing scope don't down that all cheapo scopes are horrible! My only problem with the scope is that the mount is a POS the case of this scope yours was 80 bucks cheaper than mine online

My question to you would be, "What exactly is your experience with using scopes of much better quality then the 'el-cheapos'? Cheap scopes ar J-U-N-K. ANYTHING made in China is J-U-N-K. Drop some coin on a scope of higher quality and tell me that you can't notice any difference between that and BSA, Barska, NcStar, UTG, Sun Optics, etc., etc., etc.......... Clarity, tracking, eye relief, FOV, paralex,  holding zero (espeically when using a variable), over-all quality/construction is something you can only appreciate if you're familier with it.

I'd have to say it is not barrel temp that effects accuracy but changing temperatures that do.  Either you have to shoot cold or as near as to a cold barrel as possible, or get the barrel warm and keep it there for awhile.  A barrel and the rifle will expand and contract differently as it warms and cools which is what "pulls" your POI around.

Cold shots are the typical way to sight and test a rifle set up because that is the most common application, cold first shots with backup shots.