I had been wanting to do this for about a week now, and today it worked out. It is also coincidentally synced with a comment in arcwelder's lightweight carbine thread.

As readers likely gathered from the title, the tolerances of the test were... loose.

At a 300 meter shooting position, a mag loaded with five rounds 77gr OTM, ten rounds 55gr FMJ, and five more rounds 77gr OTM is loaded an made ready in an AR15 with a non-floated 16" gov't profile, button rifled, chrome lined barrel with rifle gas and a Surefire SOCOM556 RC2 attached. The rifle is equipped with a well-used GI trigger and a Leupold 6HD Patrol. The 77gr OTM 5-shot groups are fired from the prone position, with the forearm of the rifle resting on a backpack, and the stock being supported in the shooter's shoulder pocket by his left hand. The first 77gr group is fired; the shooter travels to the target by vehicle, and marks the hits in a 5" spread. The shooter returns to the firing position, fires ten rounds into the dirt as quick as he can pull the trigger, and then lays down to fire the remaining five 77gr rounds. Hits are marked and measured at 8".

These results temporarily satisfy my broader curiosity on the effect of heat, in this particular gun. I'd like to do it again, on a day that lacks the breeze I had today, but I doubt it'd matter much. If anything, it'd be nice to do it a couple more times, and perhaps average the results.

As I said in that thread, sooooo much has been said on the topic. People are concerned about it, OK but are they being reasonable with all the "data" and what they think they need. I spoke to it in three different ways in the other topic, I can put it this way too:

Are you a sniper trying to shoot someone in the face, past a hostage? Are you in combat trying to put a bullet in a guy? Is this benchrest competition?

Shooting bullets is all about physics, so of course heat will do something. To get to the real answer, we each have to fill in answers for ourselves. Meaning, purpose, budget, skills, goals, and more.

I think what people are really asking when they ask about heat and accuracy, is if there is a way to "defeat" or overcome it. The answer to that is fuck no. Is there a best rifle for it? No, these are actually separate things.

That's why I'll be a broken record on - have a purpose for your rifle, be realistic on that purpose. Instead it can and has gotten distilled into "Colt makes the best rifles," of course we know that's not true, but insert maker or product of your choice. Pencil barrels are a choice, heavy barrels are a choice, billet lowers, hydraulic buffers and so on and so on.

If people are only trying to buy their way out of a problem they think they have, well, the people who sell rifles and parts have your number.. I think in online gaming and collectibles, they call this a "whale" now.

The rifle this experiment was performed with was built with the primary focus on full-time suppression and getting more terminal performance out of heavier projectiles at longer ranges (I guess that's a fancy way of saying velocity). It wasn't built for accuracy, but it does well enough in that department, to satisfy me, in my perceived use-case. The effect of heat was a curiosity; not a concern. Well, I guess if the effect was atrocious, I would be seriously reconsidering the build, but it wasn't so... cool.

Thanks for the report, and good on you for actually conducting an experiment to gather information for yourself. I long ago learned that much of what the pundits du jour had to say about some aspect of guns or shooting was incomplete, misleading, or just wrong. It’s hard to imagine where I would be today if I had relied solely on what other people told me rather than what I determined for myself.

There is information pertaining to the effects of barrel heating on point of impact and precision, including some from Bryan Litz who is a genuine authority, but the only way to have any confidence about its affect on a specific gun is to determine it oneself.

Regardless of how limited in scope an experiment may be, we are bound to learn something from it—even if it’s just ideas for future experiments or methods. An anything is therefore usually better than a nothing based ignorant supposition. I appreciate your efforts and posts about them.

quote:

Originally posted by arcwelder:
That's why I'll be a broken record on - have a purpose for your rifle, be realistic on that purpose.

Bingo

quote:

Originally posted by KSGM:
five rounds 77gr OTM, ten rounds 55gr FMJ, and five more rounds 77gr OTM
first 77gr group...in a 5" spread
ten rounds into the dirt quickly
five 77gr rounds...measured at 8"

I've discussed similar shooting a few times in the "Long Range Rifle Discussion" thread.

First off, I own only mid-contour and heavy-contour barrels. I've only talked with one precision shooter with a pencil-profile barrel. It was a high-end pencil barrel, and he said it shot quite accurately -- 1 MOA to a touch under, consistent with distance.

The accuracy on my barrels doesn't seem to deteriorate noticeably as they get hot. The worst heat damage I've done in competition was to an 18" Wilson Combat fluted Recon profile barrel. I was in the "assault stage" of a Competition Dynamics match in Douglass, WY. In a 5 minute CQB stage I consumed two full 30-round mags of ammo, plus a few more into the 3rd mag. Stopping the clock required hitting a 4-5 inch plate at around 250 yards, from an OK position on a boulder. I dialed 250 yards on the scope and got a first round hit. My suppressor was slightly smoking and the handguard was uncomfortably warm after the stage. Fortunately the barrel shot well on the 300-650 yard field stages the next day -- after a thorough barrel cleaning. A heard a few AR guys whose barrels shit the bed that day, and accuracy wasn't good the next day. All the rounds I shot in that match were factory Hornady 75 grain HPBT.

I've used multiple 16" and 18" Wilson barrels in the 2-rifle matches at Whittington Center, NM. Originally I shot the 10-50 yard targets with 55 FMJ ammo, then transitioned to Hornady 75 HPBT for the 75-450 yard targets. I always seemed to struggle with the first few rounds of match ammo, after starting with the cheap stuff. The AR portions of these stages lasted 2-3 minutes, with my round counts varying from 15 to 30.

One summer I did extensive testing with 223 ammo -- switching among brands, bullet types, and bullet weights. I found some transitions to occur with no noticeably accuracy issues, but others made me question if I even knew how to shoot an AR accurately. One of the worst accuracy anomalies occurred when I shot match-grade ammo immediately after 55 FMJ. Depending on match ammo brand, it took 3-8 rounds of match ammo before accuracy returned to normal levels. WTF flyers for the first few match ammo rounds were especially bad with Hornady ammo -- 73 ELDM, 75 HPBT, 75 HPBT Black. FGMM 69 was more tolerant of the transition from 55 FMJ, but the first shots still weren't where they should be. As a result, I no longer try to save money in matches by using cheap ammo on the close targets.

Every barrel is different in how it reacts to ammo. You might have heat issues. You might have ammo transition issues.

quote:

Every barrel is different in how it reacts to ammo. You might have heat issues. You might have ammo transition issues.

I had been shooting 62gr FMJ ammo, the previous day; no cleaning between sessions. You seem to imply that the transition effect is more influenced by the ammo you're switching to, than the ammo you're switching from. If that is the case, is it fair to say that the group size increase I saw was indeed due to heat, and not the ammo transition?

Maybe, maybe not.

My barrels don't have temperature issues. They pretty much shoot the same -- hot, cold, winter, summer, anywhere in between.

SMK 69 bullets are some of the most tolerate out there. A barrel with a reasonable twist that won't shoot them accurately has some real problems. Even in my shot-out barrels, FGMM 69 were reasonably accurate at moderate distances. So, limited issues with transitions to SMK 69 is likely due to the beauty of its design.

I've shot less of FGMM 77, as the Hornady ammo launches faster with essentially the BC. Transitions to SMK 77 from 55 FMJ resulted in some fliers before accuracy settled down.

Transitions from 55 FMJ to VMax 55 loads doesn't go well for me. IIRC about 5 rounds before VMax accuracy stabilizes. 55 VMax is similar to FGMM 69, in that if the ammo doesn't shoot well, the barrel is likely toast.

Sierra BlitzKing 55 seems to be more tolerant of ammo transitions than Hornady 55 VMax. But it still doesn't like coming from 55 FMJ.

Sierra TMK 69 and 77 bullet loads don't seem to like transitions from just about any type of ammo, even other match ammo. Hornady's 73 ELDM is in the same category. IMO the newer higher-BC bullets are finicky. But their accuracy really stinks immediately after ball ammo.

With the Frontier line by Hornady, none of the ammo I tried likes a transition from ball ammo. This includes both 5.56 and 223 loads -- includes BTHP loads of 55, 62, 68, and 75 grains.

FGMM 73 Berger shoots pretty well after transitioning from FGMM 69 and the Hornady match rounds. It doesn't like a transition from ball ammo.

Thank you for the thorough reply, as usual, fritz. I shot more of the 55gr today, so I'll make sure I group with the 77gr, before I do any shooting with anything else. Granted, I shot two mags of two different types of 55gr today, and I can't say exactly which I finished with, so the results won't be super useful.

I figured heat would be more of a factor, not only due to the gov't profile, but the combination of profile and a non-floated build. I guess the non-floated factor would result in a deviation in one direction, due to pressure exerted by resting the gun on the backpack, not an all-around opening-up. I dunno.

Is this ammo switch effect something that is as noticeable in a gun like mine, that's shooting around 2MOA, as it is in guns like yours, that are presumably sub-1MOA?

quote:

Originally posted by KSGM:
Is this ammo switch effect something that is as noticeable in a gun like mine, that's shooting around 2MOA, as it is in guns like yours, that are presumably sub-1MOA?

I can't speak for your rifles, but I've experienced the trends in my own barrels. A few of the better guys that I've shot with have similar experiences to mine.

The more accurate the rifle/ammo/shooter system is, the easier it is to determine when something wonky is going on. All my ARs have sub-MOA capabilities. When I'm not being a sheep dip behind the trigger, my better ARs can shoot around 1/2 MOA, even at extended distances.

What I was getting at was... If a 1MOA gun has an ammo transition effect that temporarily opens a group to 3MOA, would a 2MOA gun experience the effect at the same rate, or would the effect eventually be inconsequential, as the origin weapon's accuracy worsens? Would that 2MOA gun see an opening-up to 6 inches (3MOA) or 4 inches (the same 2 inch addition seen by the 1MOA gun). I suppose the first scenario seems more plausible. Surely there's a point where origin weapon accuracy is poor enough that the rule wouldn't apply the same way, but that's not a gun anyone here has anyway. I'll continue to gather my own data.

When able, I intend to repeat the same test circumstances already reported, though this time the 55gr will have been the last ammo I shot. I also intend to perform the test with solely 62gr FMJ (having primed the barrel with said 62gr), to avoid an ammo switch altogether. The groups shot for accuracy will be worse, but the rate of group increase should remain, if the heat is causing it.

Any thoughts on what it might be about the 55gr FMJ that seems to adversely affect so many different match ammo loads, albeit temporarily? Composition of the jacket maybe, like leaving something in the barrel that commercial match bullets don't like? Fouling by some chemical component of the propellant?

Going from memory, there were a few people who commented on my presentation in the Long Distance Rifle thread. IIRC NikonUser, offgrid, and Alpine -- all more experienced shooters than I, but all use hand loaded ammo. Meaning that they really don't bounce among loads with different bullets and powders. The vast majority of precision shooters in my neck of the woods also handload. Those who don't roll their own pretty much stick with one factory load.

I have experimented a lot with various factory loads, with the result of finding ammo that comes fairly close to the accuracy of better handloads. I've found some really disappointing ammo, too.

The more experienced folks scratch their heads a bit on my results. I chalk it up to similar issues when switching between 22lr loads. Now centerfire ammo doesn't have the bullet wax coating of rimfire, but there are differences of powders and bullets.

I meant to comment earlier but work got in the way.

My thought us that there are different types of powder used in the different factory loads. It is known that switching powders when looking for a load causes some issues. The astute handloader will run a couple of patches through the barrel between different powders and disregard the first few rounds with the next powder.

Well, I think, after today's brief session, that I can conclude that this weapon/shooter system is not inherently accurate enough for any of this stuff to matter. I did as I said, and shot the same sequence as before, with 55gr having been the last stuff through the barrel; I then also did the 100% 62gr sequence. Results have us in backwards land: Both first groups were 12"; the second 77gr group was 8.25"; the second 62gr group was 6". The only potential odd factor is that I had to dip into a new case of the 77gr stuff, which may be a different lot; that was what printed the first 77gr group. I don't think that, or heat, or ammunition switching, is having any consistently measurable effect on this system though.

One thing I did notice, and am curious about, is the POI was noticeably lower (at least 6"), compared to the other day. Conditions are wet and rainy today. Could cooler, wetter air have the POI change noticeably at 300m, with this type of weapon/ammo?

One more puzzling note: I have shot a 12" 5-shot group, with that same 77gr, at 530m, with this same rifle. It seems to be capable of 2MOA, which I am plenty happy with. Perhaps those 2MOA performances are flukes; perhaps it is a 2MOA gun, but I'm more of a 4MOA shooter. I certainly was today.

There are too many unknown variables for me to say much about your results, but two exterior ballistics factors to keep in mind.
Higher humidity reduces air density (despite much misunderstanding of the fact) and all else being equal high humidity = higher POI.
Cooler air, though, is denser and therefore colder temperatures = lower POI (all else being equal, of course).

As a practical matter humidity differences usually have very little effect on trajectory at moderate ranges. Air temperature is much more significant, but it’s not as if you zeroed your rifle on a southern Florida day and are now shooting in Fairbanks in February.

Well, the weather conditions should have cancelled each other out today then.

quote:

Originally posted by KSGM:
Well, the weather conditions should have cancelled each other out today then.

To a degree, but as I added above, air temp is much more significant than humidity.

I struck through “much” because neither will have much effect on something like an IMI 77 grain OTM bullet. I ran my figure of 2800 fps MV for that load through an Applied Ballistics solver and got the following trajectory drop figures using different temperature and humidity values at 300 yards. (“Drop” is independent of sight height and zero range.)

Standard atmospheric conditions (59°, 29.92" barometric pressure, 0% relative humidity): – 27.108"
Same as standard except at 100% RH: – 27.076" (I.e., slightly less than at 0% RH due to the less dense air).
Same except at 100° temperature: – 26.818" (Less than at 59 degrees, again because of less dense air).
Same except at 0°: – 27.620" (The greatest drop because of denser air, but still a minor difference from the least drop).

So, with that load no reasonable environmental conditions would have more than an inch POI difference at 300 yards.