7.5x55 Swiss Load Data and GRT

[slowworm]

This started as a question, but the more I looked at it the more I figured out what was going on. I decided to post this anyway in case anyone has a similar question in the future. I found it interesting to run down the answer, but maybe that's just me...

I'm using Gordons Reloading Tool to work up a GP-11 clone, and I'm getting results that far exceed the max loads for 7.5 Swiss listed in the reloading manuals. If I use data specifically for my K-31, using my Norma brass case volume, my trim length and my OAL based on measuring a 175 SMK in the chamber with the Hornady OAL gauge, I get a charge of about 43-44 grns of IMR-4895 for a muzzle velocity of around 2500 fps. Pressure is just below Pmax-25%, so all should be well.

On checking that with Hodgdon data, that seems way, way hot. Likewise Sierra data. I'm comparing to H4895, which isn't exactly the same, but good enough for a sanity check. Apparently I'm going to blow my head off, being about 4 or 5 grains over max.

I enjoy my eyes, ears and having all my fingers work, so I decide to do some cross checks. I enter that Hodgdon and Sierra data into GRT and get and MV close to the published data, but very low max pressures, about 50%-60% of Pmax. Hmmmm.

So I then enter my 308 pet load with IMR 4895 and 168grn SMK into GRT, which I have real data for, and get a max pressure of about Pmax-15% and an MV of 2670fps. Actual measured MV is about 2640 fps, so pretty damned close. GRT seems to be working just fine. 308 Win and 7.5x55 aren't so different, GRT is known good for one, so it must be good for the other, right?

So what's up? More digging around and here is what the story is. 7.5x55 will chamber and fire in the original M1889 rifles. These are not nearly as strong as the later M1911 and K-31 models. So the published data is safe to fire in any Swiss rifle, where GP-11 is not. Off to Swiss Rifles we go...

Schmidt-Rubin FAQ (swissrifles.com)

Q. Can I fire GP11 or other 7.5x55 rounds in my 1889?

NO!!! The action on the 1889 is too weak. The Model 1889 action was designed to handle rounds that generated at most 39,000psi. GP11 rounds produce 45,500psi. I cannot say just how much pressure INDEP or Norma ammo generates, so I cannot recommend using either round in an 1889 action. It safe to fire modern 7.5x55 rounds in 1896/11s, 1911s and K31s. If you wish to fire your Model 1889, you are going to have to reload your own cartridges.

Ah ha! So I go back to the published data entered in GRT and the max pressure is about 35000 PSI. So safe to fire in an original M1889. it all makes sense now.

I'm off to work up a GP-11 clone based in GRT data now.

J.

 

[biffj]

I know this post is a bit old but you may find that Reloder 17 and 175gr TMK will give you an excellent clone for the GP11 round. You will need to seat the bullet a bit shorter than the overall length shown in manuals as the shape of the GP11 bullet is much different. I've been running about 48.5gr of RL17 for the Swiss rifles and getting very good accuracy and reliability.

Frank