Is this normal?

[Dirt_Nap]

I'm doing load development on a custom 300prc. Deviant action, proof cf barrel, manners stock. I've been shooting all virgin ADG brass, h1000, and Hammer 199gr. I've shot approximately 53rds. I've done pressure test successfully and I'm starting to shoot 3rd groups. It liked 79gr and 80gr. (First pic). Test the same charge a few days later and groups open up (second pic). I cleaned the barrel and groups are tight again. I'm only getting 10rds give or take and everything opens up. I know h1000 isn't dirty but is this normal with a new build? I'm trying to figure out why this is happening

 

[The_Whitetail_kid]

Pretty standard practice with new guns. They need cleaned often.

 

[Dirt_Nap]

Pretty standard practice with new guns. They need cleaned often.

Generally when does that start to mellow out?

 

[Predator 22]

I had the same problem with a benchmark barrel. After about 120 rounds it settled in and is consistent. It is a 6.5x55 Swede so may not take as long with a prc.
I figure my first hundred rounds are for testing seating depth and fire forming brass with maybe a little powder testing and rough zero. Some barrels come around faster than others for me. Most of my custom barrels don't need much break-in. All my factory barrels were copper mines for a while.
I'd clean it every 10 or so rounds. You will notice a difference when it gets smoothed out.

 

[The_Whitetail_kid]

Generally when does that start to mellow out?

I feel like it depends on the gun but I bet 70 or so and it's getting more consistent.

 

[Eric H]

Clean it until those loads are consistent. Great grouping. Had sub .5 MOA in my .308 today on 4 shots but I will go back another day and see if it was a fluke or will remain consistent.

 

[Dmagna]

Fireform 100 pieces of brass with cheap bullets and a consistent powder charge. Chrono them to see if you are getting consistent speeds or if there is a speed up.

 

[SSG Graybush]

If it continues after barrel break in, 80-120 rds and barrel speeds up 30-150 fps, consider the scope maybe causing the poi shift. I like to use a known good scope for load development, and to evaluate a new barrel. I've seen a bunch of bad new scopes, from SWFA, Athlon and even a new Gen 2 Razor. And make sure your parallax is focused and mounts are torqued. These are the most common issues I've seen at range.
I like to fire form my brass as stated above, do a ladder test, find a charge weight to start with and max load. Plink the rest till the barrel speeds up, then redo ladder and start the load development. There's no set number of rounds, well machined/lapped barrels take less rounds to break in and usually speed up less.

 

[Bob Wright]

Notice you are getting a lot of vertical so, that's velocity changing. Plus, your brass is fireforming messing with velocity a bit. Don't worry until your brass is all thru its first firing. It should improve. Be patient. Got a chrono?
Of course make sure everything with scopes, mounts and take down screws are tight.

 

[243winxb]

Copper deposit build up is hurting accuracy. Barrel may need more Lapping.

Clean with an aggeessive copper solvent. Barnes Bullets Cr-10 Bore Cleaner.
Barnes Bullets started this many years ago. Copper Fouling. The answer was cut more grooves in the bullet. Helped some & also reduced the high pressure problem when using load data for lead core bullets.

Or change to Sierra bullets.

Virgin brass. Case volume will change after the first firing. An adjustment to load data is needed.

3 shot groups are almost always smaller then a 5 shot group. When i think i have the perfect load, i fire 4 groups of 5 rounds each. The average tells me the true accuracy of gun & load.

 

[Flintlock shooter]

I had the same problem with a long barrel 6.5x55 earlier this year. I was using H1000 and it was copper fouling. Switched the powder to H4831SC and the problem was extremely less. Maybe try a different powder, one that is a little bit faster burning.

 

[Mikecr]

On the surface I suspect scope issue or fouling change.

 

[bultinkle]

Up your sample size and see if it still repeats

The multiple 5 shot groups is good, make sure to consider your poi and extreme deviation from it.

 

[C-130 Dude]

I'm doing load development on a custom 300prc. Deviant action, proof cf barrel, manners stock. I've been shooting all virgin ADG brass, h1000, and Hammer 199gr. I've shot approximately 53rds. I've done pressure test successfully and I'm starting to shoot 3rd groups. It liked 79gr and 80gr. (First pic). Test the same charge a few days later and groups open up (second pic). I cleaned the barrel and groups are tight again. I'm only getting 10rds give or take and everything opens up. I know h1000 isn't dirty but is this normal with a new build? I'm trying to figure out why this is happening View attachment 486647
View attachment 486648

How many rounds total through the barrel? How many did you shoot in breakin?