7mm RM - Higher Pressure, Lower MV Mystery

[Shootin4fun]

Hi Guys,

I've been having some problems with pressure in my otherwise great shooting Savage 7mm RM so yesterday did a very controlled test comparing loads in 2 cases. Before giving the details, I'll sum up by saying in one brand of cases I'm observing higher pressure signs, but LOWER velocities than another brand of cases.

Okay, the details:

All cases prepped the same way.
Hornady vs. WW Super.
ProChrono used to measure mv.
120gr & 140gr TTSX
IMR4831, all cases individually weighed; I weigh to +/- 1 kernel as best I can with my 1980's classic RCBS 10-10
CCI250M

Hornady primers show more of a roll to their perimeters, WW Supers pretty flattened out. 1 of the 140gr rounds was much hotter for some reason (maybe due to sitting in warm chamber for 45 seconds before firing?), and the bolt was totally stuck. Had to mallet it open. Multiple groups fired over the course of the day, alternating cases and weights. Barrel heat not a factor.

Drum roll please...

Hornady rounds clocked in about 100fps higher in both 120 & 140 bullet weights! I always thought all other things being equal, higher pressure results in higher velocities. Of course if changing primers or powders we can see higher pressure with lower velocities, but that's not the case here.

Can someone explain why I'm seeing this and how it can be?

 

[FearNoWind]

https://gundigest.com/gear-ammo/reloading/bullet-ballistics

 

[Shootin4fun]

https://gundigest.com/gear-ammo/reloading/bullet-ballistics

Yes, I know all of that but it doesn't answer my question.

 

[g0rd0]

how far are your bullets set from the groves? Too close could cause higher preasure.
Are you using the right powder,,, IMR7828 with a 140 to 160 grn bullet is fantastic but IMR 3031 is not a good choice. You can use a fast powder like 3031 but it burns hot and preasure spikes too fast for 7rm.
I realize that comparing 7828 with 3031 is an extreme comparison but, seating depth and powder choice means everything in 7rm.
I have a savage 111 in 7rm and it is not fussy about what I put in it (with exception to Remington ammo and brass), and it really shines in the 140 to 154 pills with IMR7828

And it is really fussy with brass, loves federal, tolerates Winchester absolutely despises Remington (bolt lift problems with Remington) so just do not use Remington. I have even tried factory loaded Remington and had bolt lift problems while my buddy has a 700BDL and there are no issues with the same ammo.

 

[Shootin4fun]

Thanks for the input. I've found loading for this 7RM to be more difficult than the other non mag cartridges I load. That said I rarely get ES or SD in single digits.

I'm far off the lands, as these are Barnes. I never run anything closer than .010 anyway.

Interesting regarding the brass preference. I've had good luck with IMR 4831. I was thinking of speeding it up to H4350 with the 120s but maybe not now that you say that. I have slower powders like RL 22, 25, and H1000. It's strange because I'll get a run of fairly good ES, but then a few hot ones now and then. Barnes recommends RL22, but I hear that it's temperature sensitive.

I think more importantly, you may be on to something...maybe my Savage doesn't like the WW Super brass with the 4831. Maybe it is thicker and/or more brittle. I have had none of these pressure problems with Fed or Hornady brass. I dropped my charges from 62.9 (very accurate, 3200 fps 140gr TTSX) to 61.5 which were fine with Hornady and Fed brass and gave higher velocities but was popping primers and blew 2 ejector spring pins when using these WW Supers. That's the weird part; higher velocities and less pressure signs with the other brass. At 59.5 with the 140 TTSX they're right on the edge of over pressure with the WW. They were older, and came in boxes, made for reloading.

I recently had a brake put on this rifle, and I guess it's time to go back to the drawing board anyway. I have not been getting as tight groups, but should be getting better since the recoil is greatly reduced.

 

[Lefty7mmstw]

Oft times stiffer brass seems to be lower pressures but is not...
I suspect hornady brass is a bit stiffer and is showing a bit lower pressure as far as signs than ww brass is... Put a strain gauge on it if you want any clue as to what you are really doing...

 

[Shootin4fun]

Good idea lefty. I always seem to get higher than expected velocities for a given powder charge when I hand load for this gun. Factory ammo comes out as advertised.

So, what I've decided to do is start over with unfired WW (which also has a .015 shorter shoulder than my chamber) and drop the powder charge 1.5 grains below my lowest charges to 58.0 IMR 4831, and use some 139 SSTs just to fireform the brass to my chamber. It seems to me that SSTs also usually produce lower pressures than Barnes. I'll monitor speeds to see how all that affects things. Then work up Barnes and Bergers from there. Picked up some 168 Classic Hunters at Cs on sale. Should be fun.

Upon reexamining my records, turns out my earlier statement about consistently lower MVs from the WW that shows pressure signs is not entirely correct.

This brass is old, I can tell. It came in 20 round boxes that have the cardboard partitions between rounds, you know, before plastic was invented. lol I remember when we used to reload and use the factory boxes for finished rounds instead of the plastic 50 or 100 round boxes we now use.