Lower than expected velocity from a 7x64 Brenneke.

[jbmauser]

I've been doing some load development for my Zastava 7x64 Brenneke and the velocity is quite a bit lower than what the reloading manuals list. Now I realize I'm not going to get exactly what is listed in a book but was hoping to be a little closer than I am.

I'll start with the components. Barnes 140gr TSX, Speer nickle plated and primed brass, RL 22, H4350, and H4381 powders have been tried.

The rifle has an extremely long throat. I haven't done a chamber casting but even with the long TSX bullets I can't seat them out far enough to touch the lands. Basically I've set my COL at the max the magazine will allow which is way longer than the reloading manual specs. I also suspect a generous chamber because starting and mid range loads leave the primer protruding from the back of the case. Even with max loads the primer is sticking out very slightly. Barrel length is 22 inches.

The load data I've been using is straight up .280 Remington data from the latest Barnes manual. The manual lists velocities around the 2950 fps range with the three powders I've tried from a 24 inch barrel.

The velocities I'm able to get avg around 2650 -2675 fps with all three powders. Accuracy is within my acceptable limits with RL 22 showing the most promising groups.

I know it will get the job done at the velocities I'm obtaining but I was kind of hoping for a bit more out of it. There seems to be absolutely no pressure signs at the max loads and I'm basically getting 7x57 velocities out of it.

Any suggestions or experience with a situation like this?

 

[Gatehouse]

first , make sure your chrono is working properly. If so, then keep working up a little bit at a time until you get the velocity you want.

 

[jbmauser]

My chrono is pretty much brand new (not that that guarantees anything). I should fire some other rifles over it to make sure it is working ok. I have some loads that have been fired over a different chrono so that would be a good start and see if the numbers match up.

 

[farshot]

with my brno 7x64 (edited to make sense) I crowd 3000 with RL19 and 140 gr Hornady sst or Nosler BT...for what its worth.

 

[bigedp51]

What was your Reloader-22 load or charge for your 2650 -2675 fps ?

Quickload data below
(54 grains of Reloader-22 = 2676fps at 41,315 psi chamber pressure)
I need case trim length, and OAL to get more accurate data.
Your primers are protruding because your at 30-30 Winchester pressures of 38,000 cup or 42,000 psi


Cartridge : 7 x 64 Brenneke
Bullet : .284, 140, Barnes 'TTSX'BT 28474
Useable Case Capaci: 60.027 grain H2O = 3.897 cm³
Cartridge O.A.L. L6: 3.307 inch = 84.00 mm
Barrel Length : 22.0 inch = 558.8 mm
Powder : Alliant Reloder-22

Predicted data by increasing and decreasing the given charge,
incremented in steps of 2.0% of nominal charge.
CAUTION: Figures exceed maximum and minimum recommended loads !

Step Fill. Charge Vel. Energy Pmax Pmuz Prop.Burnt B_Time
% % Grains fps ft.lbs psi psi % ms

-20.0 86 48.00 2354 1723 29600 9799 84.1 1.440
-18.0 88 49.20 2418 1817 31615 10169 85.7 1.402
-16.0 90 50.40 2481 1914 33780 10533 87.2 1.365
-14.0 92 51.60 2546 2015 36110 10889 88.7 1.329
-12.0 94 52.80 2611 2119 38616 11235 90.1 1.293
-10.0 97 54.00 2676 2227 41315 11570 91.4 1.258
-08.0 99 55.20 2742 2338 44224 11892 92.7 1.223
-06.0 101 56.40 2809 2453 47366 12198 93.8 1.183
-04.0 103 57.60 2876 2571 50744 12489 94.9 1.146
-02.0 105 58.80 2943 2692 54436 12761 95.9 1.109 ! Near Maximum !
+00.0 107 60.00 3010 2817 58419 13012 96.8 1.073 ! Near Maximum !
+02.0 109 61.20 3078 2944 62741 13242 97.6 1.039 !DANGEROUS LOAD-DO NOT USE!
+04.0 112 62.40 3145 3075 67439 13448 98.3 1.006 !DANGEROUS LOAD-DO NOT USE!
+06.0 114 63.60 3213 3209 72547 13629 98.8 0.973 !DANGEROUS LOAD-DO NOT USE!
+08.0 116 64.80 3281 3346 78148 13782 99.3 0.942 !DANGEROUS LOAD-DO NOT USE!
+10.0 118 66.00 3349 3486 84222 13906 99.6 0.912 !DANGEROUS LOAD-DO NOT USE!

Results caused by ± 10% powder lot-to-lot burning rate variation using nominal charge
Data for burning rate increased by 10% relative to nominal value:
+Ba 107 60.00 3179 3142 71583 12949 99.9 0.986 !DANGEROUS LOAD-DO NOT USE!
Data for burning rate decreased by 10% relative to nominal value:
-Ba 107 60.00 2789 2418 46934 12308 88.9 1.185

You won't see flush primers till you get above 45,000 - 46,000 cup. (50,000 to 51,000 psi)

 

[jbmauser]

Thanks bigedp51! That's a big help.

So my case trim length is 2.510" (new unfired Speer cases)
OAL is 3.375 which is way out there but I was trying to seat it out as far as the mag allows to make up for the super long throat.
Charge was the max listed in the Barnes manual for RL 22 of 57grs.

So provided my chrono is accurate, and I'll be checking that in the next day or two, I should be safe to go up a couple more grains.

 

[9.3mauser]

The long throat is causing the lower velocities and pressures. I have the same situation with my 7x57, and my loads are consistantly 2-3 grs. over book max. with no pressure signs, and the velocity is higher than one would expect from a 7x57 as well. You simply have to keep going upward with the load development (carefully) until you where you think you should be.