[IPSC_GUY]

All Right I got to the range today under good shooting conditions, slightly above freezing and NO WIND finally. So I tried out the SSA 110 grain Barnes TSX and after sighting in for the round this is what I got.

First shot was always DEAD on. The second shot was almost on top of the second and the Third would always be within 1/2 inch of the first two... so three rounds down range and we are sitting at 1/2 minute of angle.

Sounds good right? Hold the phone kids, the fourth and fifth rounds were consistently high and any where from an inch to an inch and a half high above the other three rounds.

Time between each shot was roughly 20 to 40 seconds, enough for me to get up from the bench and retrieve the fired casing, look through the spotting scope and Sit back down and sight again.

So does this sound like barrel heating? Or... is something else going on?

IPSC_GUY
SIERRA II ALPHA

 

[constructor]

Was this from a clean barrel every set of 5?
Bullets act differently to the amount of fouling in the barrel.
6.8 barrels should be .277 groove and .270 bore. Most 6.8 bullets are .276-.2765 dia. so the first bullet you shoot will have some gaps for gases to go around, after the barrel get a little carbon in it the gaps seal and your velocity will increase and make your point of impact higher.
After the breakin period and the barrel stops copper fouling I stop cleaning to bare metal, I just run a patch through to knock out the loose stuff. I had to learn to clean to a certain level not clean all the way to bare metal. When my accuracy falls to more than 1 Moa on a precision barrel I clean but not until then.

 

[IPSC_GUY]

Ok I was being a bad kid and did not clean ANY till I got home. This is plumb interesting, so the level of fouling in the barrel will dictate a certain shift in the POI? Is this why Moly coated bullets are loved and HATED by some shooters?

IPSC_GUY
SIERRA II ALPHA

 

[ArtFWTx]

I found a couple of articles. Not sure if they apply.

http://www.shootingsoftware.com/measure.htm

http://www.6mmbr.com/verticaltips.html

 

[constructor]

There are always good articles on 6mmBR, I agree with Speedy on cleaning barrels when they are new, after breakin the main thing is consistency it either has to be very clean for every shot or dirty. If you clean to bare metal that cleanliness is only good for one shot and every shot after changes the fouling in a bore faster than from 40 -80 does.
I shoot every custom barrel before it leave the shop, I have noticed my velocities climb in the first few shots on a clean or new barrel.
I didn't ask if it was a lightweight barrel or not. I believe barrels do heat up and change POI but not so much in 5 shots but, from the first to the 10th in a minute yes.
I can't waste much time when testing so I usually squeeze off about every 2 seconds and usually get off 5 groups of 5 in a minute from 3- 10 rnd mags, as long as they are MOA or better I call it good.
When I work up loads I start quick like that then to hone it down I'll do the round robin and take my time.
Moly is funny, I had a barrel made by a top smith a few years ago while I was on the road. He said he put 40 -60 rnds through it with surplus ammo trying to get it to cycle, he didn't clean it(found all this out later). I didn't look at it before I went out I assumed it was unfired. I loaded 44gr of H4895 behind some 155gr Lapua Scenars (moly). I fired 2 shots at 50 to get on paper then 5 shots went into 3/4" at 100, off of POA but I don't adjust in the middle of a group. The next 5 were 1- 1-1/4", the next larger, the next larger. the barrel never shot right after that first 5 and to this day I don't know if the moly was building up in the barrel or not, I have borescoped the barrel but can't really see anything, a little grey in the corners but nothing major. That thing has been a thorn in my side for 3 years.
I have been shooting naked bullets since the late 70s, some shoot some don't but I have never had anything like that ever happen before or since.
On the other hand I built a 223 heavy Krieger for a guy in Texas a few months ago, he shoots 70gr moly Bergers between
.2 and .3" out of an AR, he says he is smoking everyone, no doubt with shooting like that.
I guess some know how to shoot moly and I am one that doesn't.

 

[Test_User]

You said the 4-5 shots are consistently high. DO you mean every group?

Do you shoot more then 1 group per range session?
What barrel is it? Profile?
Fouling shots before the groups?
How are you loading them?
As you go pick up the fired case are you leaving the next round cooking in the chamber?
As you fire more round the temp of the chamber is increasing and leaving the next in the chamber is making it hotter.

If this is happening with multiple groups then it doesn't sound like it is from a bare barrel as the next group should have been better and better with each group there after. But how do you clean and what is your schedule?

Try hand loading each round and see what your groups are. Make sure to shoot a few fouling shots first.

Letting barrel cool down or timing your shots in a group only works if you are not autoloading. With autoloading it is doing the exact opposite.


Its a double edged sword: Typically you are dealing with a thinner barrel, rounds cooking in the chamber if you wait and if you do not a thinner barrel that will walk some if its allowed to heat up. All round have to be loaded the same as if you hand load one then auto load the next it can pull a group to.

 

[joshuades]

An odd question here. Does the SSA powder get affected by temperture?

IPSC guy, when you were getting up and checking on things, was there a round chambered in the rifle? I only ask because you said it was just above freezing and your POI was progressing higher on the paper. The rounds sitting in the chamber would get heated while sitting in there and if the powder was sensitive to temperature, it could increase velocity.

Constructor's explaination of the barrel fouling and blow-by could be all there is to it, but your statement about how cold it was started turning cogs in my head. Personally, on my rifles, after the barrels stop copper fouling I don't even put a brush down them anymore, just patches. I also leave a film of oil down the bore (humidity is a killer down here, it even creeps in houses with A/C).

 

[Test_User]

All ball powders are signficantly effected by temp. Hence my point about leaving a round cooking in the chamber while trying to take time between your shots. Auto loaders are not suppose to be super consistent. To do so take some mod from what is considered normal shooting for groups. Also some barrels simply walk. It could also be the barrel nut. The fit of extension and receiver face. Anything that heat and expanding metals can effect.

I know a few weeks ago or maybe a bit longer there was another post/thread like this one. The recommendation was to shoot the groups by hand loading each one so its loaded the same and not cooking in the chamber.

From pressure testing even with a couple of fouling shots the pressure from round 1 thru caused a rise in pressure well above 1K PSI. This is why with any tests I single load each and every shot. My observations with my testing is also substantiated by others that have tested the effects of temp from rounds time in chamber on powder, primer, and effects on performance.

Not saying this is what it is but its one of numerous possibilities.

If it happens consistently every time with groups using different ammo combos then I would look at what you are doing the same each time or the gun itself.

 

[IPSC_GUY]

You said the 4-5 shots are consistently high. DO you mean every group?

Yep shots 4-5 are consistently high.

Do you shoot more then 1 group per range session?

Today I shot 5 groups.

What barrel is it? Profile?

LMT MRP 16 inch Chrome Lined.

Fouling shots before the groups?

I did run one Group of 5 rounds of S&B down range at some Dirt Clods.

How are you loading them? As you go pick up the fired case are you leaving the next round cooking in the chamber? As you fire more round the temp of the chamber is increasing and leaving the next in the chamber is making it hotter.

AHHHH HAAA ! ! ! ! For one group I did leave the rounds in the chamber and on the next I did not, simply firing one round (loaded from a magazine and then letting the bolt lock open to get more airflow) at a time. The group I fired one round at a time was tighter.

If this is happening with multiple groups then it doesn't sound like it is from a bare barrel as the next group should have been better and better with each group there after. But how do you clean and what is your schedule?

I generally do not clean at the range, only when I get home. There I use an OTIS kit with the pull through cable. I try to clean it to bare metal. Guess I am going to stop that.

Try hand loading each round and see what your groups are. Make sure to shoot a few fouling shots first. Letting barrel cool down or timing your shots in a group only works if you are not autoloading. With autoloading it is doing the exact opposite.

Its a double edged sword: Typically you are dealing with a thinner barrel, rounds cooking in the chamber if you wait and if you do not a thinner barrel that will walk some if its allowed to heat up. All round have to be loaded the same as if you hand load one then auto load the next it can pull a group to.

Ok this explains ALOT ! ! ! I guess in all honesty that I should be happy as a clam that the first round cold of each group was right where I wanted it. Thanks for the help guys.

IPSC_GUY
SIERRA II ALPHA

P.S.

Also some barrels simply walk. It could also be the barrel nut. The fit of extension and receiver face. Anything that heat and expanding metals can effect.

This also brings up an interesting point. This barrel as compared to the 5.56 barrel I have, does fit looser in the upper. Not much but it is. I actually thought about shimming it somehow. I have a love/hate relationship with the MRP, really I do.