Hmm head scratcher?

[Phil McGrath]

First the rifle details, M40a1 clone. Barrel is a 1/10 Obermeyer 5R with Obermeyer Match chamber finished at 25in. Action is trued and the scope is a Lupy fixed 10x Ultra with mil-dot using M1 turrets. After Saturday’s range visit the round count on the tube is now 1300rds, too date my go too accuracy loading has been 44. Varget under a 175SMK in a Lapua case lit off with a Federal Match primer coal 2.83, just a clone load of M118LR in this rifle.

My tipical M/V runs at about 2650ish +/- @ 70ish degree’s @ sea level.

This last Saturday, I decided it was time too give the 185hybreds a try, I figured the 44. Varget load would be rather mild at 2.90 coal, with a slight deviation I used some new Peterson Match brass. What I got was a real surprise... Same load clocked in at 2750-2741 with low ES/SD temperature was a whole whopping 85*.

The Peterson Match brass weighs in at a porky 178gr and the Lapua is 174grs. The 185’s also have a shorter bearing surface compared too the 175SMK. I was figuring the longer COAL and the added weight would have slowed it down enough that both loads would have been comparable for M/V?

With the Peterson Match cases the only pressure signs I had were flat primers however there was still a clear clean bevel around the edge, bolt lift was as slick as could be. I have no complaints about the rifles/loads accuracy just wondering if I should knock off a grain and bump the coal out too 2.95 and see what shakes out?

 

[BoydAllen]

Where are you on distance to the rifling with each bullet?

 

[Phil McGrath]

I hadn’t checked as this was a thrown together load that I figured should have been pretty mild. 2.95 isn’t a problem that I have too worry about touching either the round count on the tube is at 1300rds.

Backing off isn’t a problem for me as I was spit balling on where the big boost in M/V came from. There is a 4grn difference between the brass and a 10grn difference in bullet weight, in my pee brain of thinking that shouldn’t up the velocity nearly 100fps that is unless I just so happened too get a odd lot of Varget?

 

[mikegaiz]

I have found that a given good load for a bullet/powder/barrel combo will not usually work for a different make bullet even the same weight. and throw in different brass, and a different lot of powder????? go fish.... Back to the range for testing....

 

[Kurz]

I hadn’t checked as this was a thrown together load that I figured should have been pretty mild. 2.95 isn’t a problem that I have too worry about touching either the round count on the tube is at 1300rds.

Backing off isn’t a problem for me as I was spit balling on where the big boost in M/V came from. There is a 4grn difference between the brass and a 10grn difference in bullet weight, in my pee brain of thinking that shouldn’t up the velocity nearly 100fps that is unless I just so happened too get a odd lot of Varget?

In a sort of review:

Switch powder lots, start your test loads lower and work your way up.

Switch brass manufacturers, obtain a water capacity of both old and new brass. Peterson states that their case capacity is less than others so measure it and adjust. Smaller capacity with your same load raises pressure.

Increase bullet weight/length increases pressure even though you adjusted the COAL.

 

[Phil McGrath]

Thanks guys!

 

[NC308]

My 308 did the same thing with 175 smk and 185 juggernauts. I shot the 185 faster as well.

 

[NC308]

Not sure if you have tried the juggernauts, but a great bullet! Easy to tune and really shines 800-1000 yards.

 

[searcher]

Since you were firing virgin brass - the "slightly flat" primer issue can occur just from the loose cartridge "slamming" to the rear of the bolt face on that first firing. You might find a subsequent firing with the same brass shows no such signs. I'd measure the diameter of a fired case circumference with that of a virgin case just above the web. If fired case did not expand - I'd say you are O.K., though you really have to run the powder ladder on upward till you see real signs of pressure to know where you are.