First trip out with the 338/408 Imp.

[yorke-1]

I got out today to start testing my new 338/408 Improved. Here's the build sheet for the rifle. I haven't decided what scope to use on there yet. After the picture was taken I threw one my Mk 4s on there but it looks a little small still.

- CheyTac M310 action (made by Lawton)
- Bartlein 30" 1-9.5" twist barrel (1.350" for 5" and straight taper to 1")
- Jewell trigger
- CheyTac Chassis (McRee Big Bore folder)
- Terminator T4 brake



I loaded 3 rounds each using 300gr OTMs and H 50BMG in 1 grain increments from 130 up to 135gr. I was a little surprised by the velocities I got. These were all recorded with a Magneto Speed V3

130gr:
Avg- 2943 fps
SD- 9 fps

131gr: There was one shot that came in at 2908 which threw the numbers. The other shots were 2971 and 2959
Avg- 2946 fps
SD- 33 fps

132gr:
Avg- 2927 fps
SD- 18fps

133gr:
Avg- 2980 fps
SD- 18 fps

134gr:
Avg- 2963 fps
SD- 7 FPS

135gr:
Avg- 3047 fps
SD- 8 fps

The initial velocity with the 130gr charge seemed in line with what I expected. The following charges are what have me a little stumped. All charges were hand weighed and bullets all seated withing .002" measured to the ogive. The fact that the SD on all of the loads is reasonably small given a case of this size is really throwing me off. The loads appear to be consistent, but the velocity just doesn't seem to track the way I expect. I will say that in my experience Bartlein barrels have always been on the slow side, especially their 338s. These velocities are just a little lower than I expected even with the 30" barrel.

I shot groups starting at 100 yards to zero the scope and then moved back a few hundred yards at a time with the final group shot with the 135gr charge at 1025 yards. The group at 1025 isn't bad but could be better.


All of the groups were in the .5-.6 MOA range which is about as well as I was shooting any of the guns today (I was having an off day with all of them).

If anyone has some insight on the odd velocities I'd love to hear it. Next trip out I'll start at 135gr and work up a little more and see when I start seeing pressure signs.

 

[longrangebo]

Seems like a very light load to me! I am using 141.5 gr but with the 300 match king 3250 fps No presure but you should measure just before the base I don't like to see more then about 6 thou. from new cases !

 

[yobuck]

How many minits of elevation from a 100 yd zero to 1000 yds?
My 338 big Baer with a 34" 10 twist Benchmark needs 17 minits.
My load is 137 gr h50 bmg with a 300 smk.
Chrony claims +3200, but I don't trust chrony.
Bruce Baer says I'm where I should be with the gun. He also claims 30" is enough barrel, but frankly I question that and insisted on longer for my gun.

 

[longrangebo]

Hi Earnie, how is everything ? I am right around 17 min. also but I am sighted in for 600 yds I have the 40degre base./

 

[yobuck]

Hey Bo, I was wondering if that might be you. By way the gun you got for my friend
is at Bruces getting rebarreled as above and a Nightforce installed. Should be about
finished, but you know how that goes. lol

 

[longrangebo]

Yep, lucky to get it by hunting season ! Bruce is a very good smith and it will be worth the wait will that bolt open up enough ? I had to get a new bolt for the Lawton! Are you down south ?

 

[yorke-1]

Seems like a very light load to me! I am using 141.5 gr but with the 300 match king 3250 fps No presure but you should measure just before the base I don't like to see more then about 6 thou. from new cases !

Your velocity is right there with what I'm looking for out of this caliber. It seems like there's a lot of range in powder charges to get there though!

FWIW, the group in the picture was shot at 1025 yards after dialing up 21.75 MIN from a 100 yard zero. I'd say that it would take about 22.75 MIN to be on at 1000 yards. For that to make sense either my velocity or the BC I'm using is way off. I used a G7 BC of .412 for the 300gr OTM because that's the number that's tracked for me in the other .338s I run that bullet in.

 

[yobuck]

Possibly some difference between powder lots causing differences.
I was given the max oal for the 300 smk for the gun by the smith.
Then told to load 1 round starting at 132 gr and 1 each with 1 gr increases.
Shoot each over a chronagraph till it says 3200 or pressure is determined.
Then stop and look for best accuracy by tweeking depth and small powder increases.
I have been to 139 gr but didn't see the merit for the increase over 137.
FWIW the 300 smks and the 300 otms and the 285 hornadys all shoot well with my
load. Really not much differences in them out to 1600 which is as far as ive shot the gun to this point.
With all the people ive known over many years, ive not known any id believe, who claim
to have killed a deer in PA at a mile. So I see little point of shooting any further than what I do.

Bo yes I'm still in Dixie, but starting to feel a little itchy to leave.
Lowell made up some new eyepieces for the Kowas I'm anxious to see and try.
Larry has a set. Ill send you an email.