need help with 338

[7.62_Reaper]

the pic below shows my groupings take today, i shot at a range of 100 yards with my 338LM. and i have been having trouble getting the groups better but can't seem to get the right formula for this rifle. here is the break down for today shoot.

rifle:
338LM
stiller tac 338
benchmark 28" barrel 1:9.5 twist
jewell trigger
manners T5A stock

ammo: (same for all 4 groups)
300gr Sencar set .02 off the lands
lapua brass
cci primers
90 gr retumbo

my first group was the center, i measured the muzzle velocities. 1st read 2773fps, 2nd 2763, those to are the bottom shots in that group, 3rd read 2733 (POI shifted 1.25"up), 4th was 2742 and the last was 2738. Why would the MV drop steadily after the bore heated up??? i thought that it would be the opposite??? and why the drastic change in POI??? after as you can tell the group's POIs stay in the same area but still no dice on a .5 MOA grouping that i am looking for.

how would i prevent that kind of POI shift and what do i do to keep the MV relatively the same (with in reason) and not drop off like that???

all help is much appreciated cause I'm at a loss thanks

 

[59..baba]

It could be a problem of distance ... 100 yards (91.4m) is probably not enough to do load development in big calibers ( and big bullets) , even at 200 m ( 218 yards ) my groups are worse than at 300 m (328 yards ) in 338LM . Try to have the best velocity with the smallest group and test it at long distance ... It's probably the best solution .

 

[oneshot.onehit]

7.62 Reaper

Every rifle is a little different, so

I had better luck with H1000 then Retumbo with the 300 grain Scenars.
And I am running them 50 thousands off the lands, also I have no problems with groups at 100 or at longer distances, a very accurate rifle for the most part.

Around 2400 rounds of 300 grain Scenars down the barrel so far.
27" 10 twist - suppressed

Cheers

 

[ahhshoot]

+1 on the H1000. Also try dropping your charge down a bit. I am getting excellent groups with just 85gr of H1000 behind Berger 300 OTM.

Also, how many rounds do you have through that Benchmark?

My friend has a Benchmark on his 6.5 CM and it took about 150 rounds to settle down. I don't know why. He hasn't cleaned his bore yet after over 250 rounds and his gun gets increasingly accurate and consistent each time we go shoot. You might want to try letting your bore foul up for a good number of shots before you expect some consistency. I have read others on here say the same about Benchmarks. FWIW, my buddy's gun is now a tack driver, I think they just need some rounds down the tube to settle down.

H1000 is a pain to get right now so if you are set on Retumbo, I would still try a more moderate load. Remember what you're shooting, and the BC Of those bullets will keep you supersonic past a mile even with a lower MV around 2500 in most conditions.

 

[Grizzdude]

Are you giving the barrel enough time to cool between shots? Same happened to me with the velocities on my 338 lapua when I was shooting say every 30-45 seconds , now I let it sit for 1.5 to 2 minutes between shots.

 

[natesguns]

H-1000. My badger ordnance doesn't care much for the retumbo. Try 89 grains of h-1000. Also to restate. Forget the 100 yard groups. I just about sold my rifle trying to make it shoot one hole goups at a 100 yards. I'm using berger otm 300 grain tactical and have had very good results If you'll send me a pm with your contact info I'll send my ballistics data to you. Don't give up. This is a great round. I found that retumbo is dirty. Not saying it isn't a good powder, just didn't work for me.

Load: 89 grains off h1000
Bullet 300 grain berger otm's. Set .030 off lands. Mine won't clear my mag. Single feed only
Brass lapua
Primer: federal magnum
Velocity: 2750
Note: When it starts getting warm outside I notice a slight increase in bolt lift. I would recommend starting a grain or so lighter.

 

[natesguns]

Here's my info. Thought this would be faster.

Good luck

 

[7.62_Reaper]

thanks for all the help

I'm going to change up a little and try H1000 instead of retumbo. should i do another ladder test?

 

[Grizzdude]

thanks for all the help

I'm going to change up a little and try H1000 instead of retumbo. should i do another ladder test?

My ladder test was a waste of 30 rounds of ammo, I then just shot groups at 300 yards till I found a good load.

 

[xdeano]

with a 28" barrel there should be a barrel node in that 2750-2780fps range i'm guessing. My node is at 2735fps in a 26".

Deano

 

[xdeano]

double post.

Deano

 

[Glenn]

I can attest to the Benchmark barrel taking a bit to get going!
A lot of copper at the start.
I just keeping shooting and it improves all the time.

Get on paper at a 100m then do all you development at 300m.

Glenn

 

[remr5]

I had great experience with using the OCW formula from Dan Newberry for load development.

 

[Blackops]

My load in my 338 LM improved is 90 grains of H1000 with 300 gr SMK seated 30k off. Super slow 2670 fps but very accurate, federal 215 match primer