Post 1...new to the AR world. So, I recently purchased a PSA Freedom upper with 16 inch SS 1:8 FF with standard/govt profile. For several months I've been testing various types of ammo at 100 yards, keeping it under .50/round. I hit the jackpot on my most recent trip to the range. 5 shot sub-MOA group with Hornady Steel match 55, much to my surprise. BTW, this was the first group of the day.

My question is regarding heat and the following 4 groups with this ammo. The groups following this were 1.1", 1.25", 1.3", and 1.5". While I'm pleased with the results, my questions are...1) is it "normal" for barrel heat to double group size after 20 rounds shot every 5 seconds or so? 2) How significant would a hbar or bull barrel help in this situation, generally speaking? For example, would an hbar or bull not start to open up until. lets say, 50 rounds or so?

Thanks/Mark

Let it cool off again and see if you can replicate the group sizes.  Lots of variation from shot to shot and group to group.

All barrels can change POI upon heating up, I have two guns with that exact same barrel on them and they will open up a bit with prolonged shooting.  As said, let it cool down between groups and they should come back in, also make sure and note the variables that may be happening in your course of shooting, weather and wind can make a difference over a period of time.

Glad you found something that shoots well for you.

That sounds about right for Hornady Steel Match.  Molon got slightly better results in his test, but his test rigs have known consistent precision.

Mark, I have that exact same upper on my scoped rifle. Yes, after about 20-30 rounds in quick succession the groups open up a bit. It is definitely a slow fire barrel. Those are good results regardless with that ammo. I have only put AE xm193, M855, and Black Hills M262 through mine so far. It did not like the AE stuff at all, but the m262 does really well. I just got in some Speer 64gr gold dot but have not had a chance to see how the barrel does with it yet. Guess I need to look at the Hornady for some good range ammo.

So far, no other ammo has even come close to the steel match. M855...might as well have shot 00 buck at the target. Next best so far is PPU 69 HPBT at 2" average. That Black Hills is on my 'to buy' list. Thanks.

If you let the rifle cool down and the groups don't tighten back up, check and see if your scope is coming lose.  I had that one happen to me before.  I was cussing at the rifle and calling it junk and it turned out to be the cheap scope rings I went with.

My results reflect yours with PPU 69 HPBT with several barrels.  Not bad ammo, but I have yet to find it MATCH ammo.  This through some barrels capable of shooting well under 2moa with 69gr gold medal match.  

Have you tried 55gr PMC Xtac 5.56 loads yet?
That stuff is hot and prints tight in all of my rifles.
I stock up on cases when it is on sale.
I use it as a benchmark for testing all my new builds or barrels.

Next range session, fire 3 cold bore shots to warm the barrel up, then shoot a group.
Let the rifle cool for 5-10 minutes out of direct sun then repeat the process for each group.
Heavy barrels take a little longer to warm up, but they also take longer to cool back down.
You wouldn't notice a huge difference. You may get a couple more groups in before it starts opening up.

To answer your questions directly, it is normal for groups from government profile barrel to open up an 1" or so at 100 if you're firing 20 rounds in under two minutes. A heavy  barrel might open up a 1/2", in contrast. I own both. A heavy barrel opens up significantly less as it heats up.

Good to hear, because I just purchased a Wilson bull barrel

FWIW

I have a 1 in 7 twist SS skinny profile Freedom upper/bbl in 5.56.  In testing various hand loads to see what it liked and how much the impact point differed between loads, I found that my groups were enlarging as the bbl got hot.  Sorry, I did not keep records of the increases in group sizes, but they'd still be soda can accurate at 100yd.  I'm normally a 5 shots and let it cool type of shooter, but got in a hurry before the rain came that day.  I never posted this, wondering if it was just my bbl, but now that others may have experienced the same thing, I know I'm not alone.  It doesn't effect my shooting, so to me it's no big deal, and I felt I got a heck of a deal when the upper was on sale from PSA.  I have NO regrets getting it an very pleased with the accuracy.

Here's my most accurate load in it.  Using my 2-7x scope set on 7x at 100yd off a bench rest, my groups are 1" or a little less.  RP BRASS, CCI #41, 26.0 GR WW748, SIERRA 55GR BLITZKING, 2.26” OAL

This, exactly...if you are concerned at all about group size, then avoid government profile barrels, they are a poor design really.

A barrel doesn't need to be overly heavy to be a good shooter...Spike's Optimal Profile CHF barrels are a perfect example, also a slightly heavier barrel from Green Mountain  (GM-M8) is a great shooter, as well as most of their SS SPR barrels.

Faxon has some great shooters too.