Sunday, January 25, 2009

7.5x55 Swiss GP-11, weighed in the balance

So many of the new K-31 owners share the same opinions regarding the Swiss GP-11 round. It always begins with: “Wow, this is incredibly accurate!”, closely followed by: “Where can I get some more of this?”

The 7.5 Swiss is an interesting cartridge. It has features that echo other notably accurate and powerful rounds over the last century. Its case is shorter, stouter, and quite tapered when compared to it’s contemporaries. The sharp shoulder and squat powder space remind me of the ‘Ackley Improved’ series.

P.O. Ackley thought we could take most standard rounds and make the shoulder sharper and the powder room a bit bigger, instantly improving performance. In almost every case it worked. The 7.5x55 might have been built by Mr. Ackley. It’s every bit as powerful as the 30-06, while also having the easy to find accuracy of the .308. In addition, it functions smoothly with the unusual straight pull K-31 action.

As usual, the measuring arsenal for this investigation includes an RCBS 10-10 scale, Mitutoyo micrometers, Central vernior calipers, some other assorted goodies, and a large cup of coffee. Starbucks beans, ground at home and brewed in a drip type coffee maker. Add a shlop of cream in a big cup, and we are good to go.

GP-11 7.5x55mm is brass cased with a magnetic steel jacketed bullet. The bullet core is solid lead with no voids, and seems very well bonded to the jacket. Several bullets were sacrificed to the cutting wheel (monkey curiosity strikes again!).

A feature of the GP-11 noticed by most people immediately is the unusual sealer applied to the bullet/case junction. Unique in my experience, the Swiss painted on a heavy band of wax. While it’s not uniform by any means, the sample I looked at had wax about .012”thick and .030” wide.

This wax seal was easily removed with nothing more than a rough towel and a bit of pressure. It’s a soft wax, even 25+ years after it was applied. Once removed it revealed a very heavy bullet crimp. Looking for all the world like a ‘LEE factory crimp’ its a very firm and uniform clamping of the case into the bullet cannelure. Many precision shooters swear by such a crimp to encourage consistent powder burn.

Two questions came popping into mind while looking at that wax seal... (A) Where does it go when the rifle is fired? Is it perchance smeared down the case walls, making extraction easier? If this is the case, why hasn’t a problem with 7.5 handloads in the K-31 surfaced, such as tough extraction? (B) Could it be possible that this wax sealer serves to center the round in the chamber? This might help account for the phenomenal accuracy of the cartridge.

It’s time to look at some case dimensions.......................

In the run up to writing this, someone mentioned “It’s going to be boring....” Friends and neighbors, why would someone say this exploration will be boring? Because... when we see the same thing every time, there's nothing interesting going on. This ammunition saves everything interesting for the range, and nothing for the loading bench. It’s so uniform that no time was wasted making graphs. If the reader wishes flat lines..... check out the average politicians brain scan.

The largest variation was found in overall loaded cartridge weight, mostly due to the non-uniform wax sealer. Weight spread was 415.5 grains to 420.2 grains. About 2.2 grains of that can be accounted for with case weight variations, and another 0.5 grains in bullet variations. The rest seems to be the wax.

Over all length measurements were the most boring of all. It measured 3.045” Exactly. No variation, *0*, Nada, Zilch, Sigh........

Case base diameter measured from .4941” to .4952”, a .0009 variation. The horror of it! Almost a thousandth of an inch! Those crazy drunken Swiss!

Case neck diameters ranged from .3360” to .3372”, a variation of .0012”. This was, of course, measured after the wax was removed. The variations seem to be due to the very firm crimp distorting the case slightly. Still minimal by military ammunition standards, and certainly not bad at all by commercial standards. There was almost zero variation found in neck diameter. They were round.

Case length began at 2.180” and ended at 2.184”, a range of .004. This measurement found no wild fliers, but was spread evenly across the range.

The bullets pulled from the case with extreme force. The first one took four attempts, till I realized a ‘pull’ was not going to do it, and a ‘jerk’ would be required. (Please hold all jokes till the end of the post!). Even so, many bullets required two or three attempts to get them out of the case. Not because they are glued in, but simply that very heavy crimp set deep into the cannelure.

The powder appears quite fresh. It’s an extruded grain not unlike IMR 4895, but with a shorter grain. In size it looks like AA2015br powder, but has the same shiny coating 'look' that IMR stick powders have.

Powder charges weighed in from 49.9 grains all the way to a whopping 50.1 grains. A 0.2 grain variation. You graph it.... I’m going to take a nap.

Once pulled down, cases were weighed. They ranged from 193.2 grains to 195.5 grains. This is a variation of 2.2 grains, and the largest spread of any single component. The cases were mirror bright inside. Berdan primed, the twin flash holes were very clear inside the case. Looking at the pristine flat base inside the case, a question came: How in the world did the Swiss form the anvil for the Berdan primer while leaving the inside of the case flat?

Using the wildly dangerous 'torch trick' to blow out the live primer (nothing too dangerous in search of data for our readers!), an extremely uniform primer pocket was found. In fact, the pocket appeared to be milled rather than punched. If so, this is incredible precision and care for a military round. The flash holes also appear to be drilled rather than punched. Stunning quality and meticulousness for a military round!

The bullet, as stated, is a steel jacketed number with a lead core. Full metal jacket with a strong boat tail. Weights ranged from 174.3 to 174.8 grains, a 0.5 grain spread. Most notable were the diameter variations: Only .0004”. The bullets measured from .3076” to .3080”, and were perfectly round. No variation at all in diameter was found.

It would be an interesting day at the range, passing a few handfuls of this ammo across a chronograph. By all reports, this is the most accurate standard military ammunition built, and frequently wins matches. Perhaps that has something to do with the general lament over the lack of GP-11. What supplies that do show up, vanish almost instantly. In fact, if anyone has a case they’d like to contribute to the cause, I’ll happily continue testing it!


Firehand said...

Yes, I have a case. No, you can't have it!

And I agree, this stuff is marvelously consistent and accurate.

DirtCrashr said...

GP-11 is great stuff! It's also still better than the Privi 7.5x55 Swiss stuff, and about the same price.

Anonymous said...

This ammo is by far the cleanest stuff that I have ever shot.... When your 3rd patch comes out clean after 30 rounds,,, you refuse to believe it... I used more patches doubting that the bore was actually "clean" than I did cleaning it in the first place...

No other surplus rifle/ammo combo will come anywhere close to that...

I wish I could find the original components to reload myself...

Anonymous said...

You will not find a more accurate cartridge for the K 31 than this one.
Even far better than the NORMA stuff i used 20 years ago.....

Anonymous said...

More of a question.........
Just bought a custom built target Model 70 in 7.5 Swiss.
I already own RCBS dies for 7.5 Swiss.
About 25% of the GP11 won't allow the bolt to close. Using the RCBS dies no resized cartridge case will allow the bolt to close.
Also I have some Hornady loaded custom 7.5 Swiss and none of those will allow the bolt to close.
Any ideas? I'm thinking step 1 would be a chamber cast! But is there a variation in 7.5 Swiss case dimensions that might be going on here?

Carteach said...

A Nony Mouse,

My suspicion is you have a min-spec chamber. The chamber cast you mentioned would be a good way to find out exactly what you have.

If that's the case, the traditional answer would be a set of 'small base' dies. I can't say I have ever seen them in 7.5x55mm.