Why?

[Unclenick]

Was123,

You should read Denton Bramwell's article on temperature sensitivity, here. It tests H4350, among others. Bramwell concludes, in part:

"If you’re not controlling barrel temperature, you about as well might not bother controlling powder temperature, either. In the cases investigated, barrel temperature is a much stronger variable than powder temperature."

So, your rate of fire and how well you cooled the barrel between shots is likely tipping the scale. Bramwell suspects the real culprit is primer temperature. Having low mass and being thermally well tied to the high thermal conductivity brass, heat from the chamber will move into the primer fast, where powder in the chamber takes some time to warm up. Bottom line, let the bore cool for five minutes between rounds. If you intend to use the gun in a situation where you can't do that, then develop best loads for the conditions you will actually use it in.

For example, suppose you want to know where a clean, cold barrel shot plus two follow-up shots will go. Set up three targets, and fire on them in sequence, putting the cold barrel shot on the first, the first follow-up shot on the second, and the second follow-up shot on the third, putting one round in each. Let the barrel cool off, clean it, and repeat. Keep repeating until you have a large enough group on each page to find the centers and compare how the center shifts from one group to the next.

Also, as mentioned earlier, group size can be a factor. If you shoot 5-shot groups, you can expect them to occasionally vary by about 50% just due to random probability. 3-shot groups can vary much more and are statistically not very useful unless you combine a number of them. You have to shoot bigger groups or at least overlay more than one 5-shot group to start bringing that down. A 10-shot group is much better than a 5-shot group, and a 15-shot group is much better than a 10 shot group when it comes to averaging and figuring what the real standard deviation is.

I'll add that there are more effective copper removers than Sweets 7.62 these days. I use Boretech Eliminator for general bore cleaning. It attacks copper much faster, soften carbon well, and is water-base, non-toxic, and odorless into the bargain. KG-12 is an even stand alone stronger copper solvent. It's only drawback is that it turns tan rather than blue as it eats copper, so it is harder to tell when it's done. But if you put it in a bore and just wait 10 minutes, all the copper is usually dissolved. A follow-up patch with Eliminator or another solvent that turns blue will verify that it's done. Note that both cleaners attack copper so fast a brass bore jag will be attacked before you can push it all the way through the bore. Plastic or nickel-plated or Boretech's special alloy are preferred with it or with KG-12.

[was123]

At the last range session, I fired a total of 30 shots in 3 hrs so I do not believe the bore was to warm with no two shots being closer than 5 mins apart. This is about my normal rate of fire when doing load development.

Thanks for the info on the Eliminator and KG-12, will definitely be buying some. Cleaning copper at the range is always a chore especially in Rem 700 in 270 Win that collects it like a miser but shoots like a dream.

[mrawesome22]

I'd still like to know if the powder and primers were from the same lot lol.

[TXGunNut]

Were you using the same rest as this spring? I have better results with a bipod with bolt guns, others have to get by with bags. Copper fouling a good suspect but last time I was in your shoes the culprit was a bit of missing rifling that somehow let go. It was truly a tragedy, hope that's not the case here. To look for missing rifling run a dry, tight patch through the bore slowly and feel for a rough spot.

[was123]

Powder same lot? Yes, it came from the same lot.
Primers same lot? I have no idea as I don't normally note lot on primers, just make and size. As a matter of fact, until about 5 mins ago, I did not realize that individual boxes of primers were identified by lot. I have 8, 4 with one number stamped on its side, 4 with another. I guess I have something new to note in my load data. Has onyone seen a significant difference going from one lot to another with primers?

[davery25]

hmm. i had the same issue yesterday...my .35" group rounds, consisting of 55grn Nosler SHOTS projectile, 25.7 grns of AR2206H, RP brass and Rem 9.5 primers just opened up to 1.5 inches. It was a hotter day then last time but this is the only thing i've noted different...

[603Country]

I've often had rifles start printing bigger than normal groups. In each and every case, the problem was either me or it was copper fouling or a combination of both. Honestly, the copper fouling is easier to fix, so first thing I do is to really get serious about cleaning the barrel. That cleaning, and my calming down and paying attention to my shooting has always put things right. However, if the good cleaning didn't fix the problem, at least I now have a clean barrel and know that's not the problem.

[black mamba]

I agree with 603Country except for one addition: loose scope mounts or rings. This will open up groups quicker than anything else.

[davery25]

is there a set of rings or mounts i should be looking out for then? the ones in question are the base rings given with the Howa 1500 rifle.

[Mike / Tx]

Two other things I have had in the past that drove me nutty just like this.

One was case neck tension. I loaded up some test loads, found my accuracy and everything was peachy. Loaded up a hundred rounds for the years worth of practice and hunting. Next time out that fall, groups weren't even within 2". Couldn't for the life of me get it down. Figured what they heck and pulled a round just to check the charge weight just in case. The bullets on some were nearly scraped shiney all the way around from the brass contracting on them where as some simply eased right out, and looked like they just came out of the box. Lesson learned for sure.

The other thing was while working up loads I was simply shooting 10 - 20 rounds, between a thorough cleaning. Similar situation, great load found, loaded up a quantity, and was happy. Couple of months later same load, same everything, groups in the 3" area. Cleaned the bore, and repeated, with same results. Got fed up and simple decided to work on trigger pull and such and after about 8 consecutive rounds the groups dropped to around an inch, then a bit smaller and stayed there. Cleaned the rifle up and back to 3" groups. Only thing I could remember was that when I did the initial work up, I also had two other rifles and possibly had switched over to allow this one to cool and not cleaned between rounds. Once fouled it shoots like a laser, but when clean all bets are off.

Similar to one of my revolvers, if you clean it up to spotless, you might as well simply walk out and shoot at least three full cylinders through it before putting it on paper if you want any reasonable accuracy. It just likes it dirty.

[Unclenick]

Match shooters have found it takes 10 rounds or so for guns to settle after changing powders, too. This is just because one powder's fouling is lighter/heavier/softer/harder than another's. So you don't change powders in the middle of a match, and neither do you expect absence of fouling to work the same as its presence. Metal fouling can cause even more effect, and some barrels build it far more quickly than others. "Copper miser" is a term you sometime hear for them.

The effect is a friction issue that changes barrel time because higher friction raises pressure, causing the powder to burn faster, which decreases barrel time by putting more of the bullet acceleration earlier in its trip down the tube, making it faster for the rest of the journey. If you find both fouled and unfouled barrel sweet spot loads, you can adjust inbetween as the fouling builds.

Another strategy is to smooth the bore so it doesn't accumulate as much fouling, as by firelapping, so fouling reaches equilibrium at a lower level. Or you can try using lubricated bullets like moly-coated or hBn coated bullets. The coatings will increase the charge weight you need a little because of the reduced friction, but it also prevents metal fouling from building as quickly or contributing as much to the behavior of the bullet.

[edward5759]

I have found temp changes will add some error to expected results. All most all rifles that I have seen after people say that they have cleaned the barrel and there is no copper in the barrel, I find a Penney worth of copper still in the thing. One rifle it would shoot 12-14" groups, the cause, Bad Primers! I only saw that a few times though.
General Purshing would have the men clean the 30-40 krag rifles with anhydrous ammonia it would darken one in ten barrels. Now there is a lot better products on the market but try another copper solvent just maybe it will help.

Edward5759

[Unclenick]

General Pershing was dealing with cupro-nickel jackets that not only fouled but built up lumps. The modern gilding metal alloy isn't nearly so bad, but it still can cause severe fouling in some bores. Since anhydrous ammonia is a gas that has to be kept in pressurized tanks, Pershing's men would not be using it. In anhydrous gas form it doesn't attack copper, anyway, which is how it could be used in refrigeration systems before freon was invented. More likely Pershing's men used what is called stronger ammonia, a supersaturated solution, or else an ammonia dope mix like that Hatcher describes, and which is also known to etch bores sometimes.

In Howe's, The Modern Gunsmith, he describes some pretty nasty mercuric compounds for removing copper deposits. He suggests it is a great money-making activity for gunsmiths, as accuracy complaints were often solved by it in his day. It would be true in some instances today, but not as much as when cupric-nickel jackets were still common.

KG-12 is the most effective modern chemical remover, but it only turns sort of orange tan rather than blue as it reacts, so it's hard to tell when it's done without patching it out, then putting something in that turns blue or green so you can see if the copper is really gone. If you really want to impress somebody, use an Outer's Foul Out and let him be there when you pull the rod out so he can see the copper on it. If the build-up is bad enough it will actually bridge and short and have to be wiped off the center rod and the rod returned before the unit can complete the job.

[davery25]

OK, the cleaning thing actually makes a lot of sense, but when i asked whether leaving my barrel uncleaned would damage it last time on 6mmbr I got reamed.

I'll try again. Seeing as I don't and can't use corrosive primers - is there any harm in just leaving my rifles uncleaned until they've fired 80 shots or so, whether thats on the same day or months apart?

[Unclenick]

No danger that I've ever seen.

80 shots plus sighters is a typical Highpower NRA match. A cleaning area is not commonly available during the match, so no, no harm is normally done by the practice. I have also run into rifles that have never been properly cleaned in hundreds of rounds. Again, no danger; just no accuracy, either. I had one Garand barrel I've griped about before that just could not go past 40 rounds without losing accuracy. No danger was present when it went past that number, but there was also no danger of me hitting the X ring at after it got that fouled.

Also, broadly speaking, it's a lot easier to get fouling out of a barrel if you get it damp with a bore cleaning product right after shooting it, but you do need the opportunity to patch it out before you shoot again. Hydraulic pressure from a bullet plowing through liquid can damage a gun severely.