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A look inside: GP-11 7.5x55 Swiss

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Zeliard
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A look inside: GP-11 7.5x55 Swiss

#1 Post by Zeliard » Sun Jun 11, 2017 9:40 pm

Originally posted by carteach0.

GP-11 Swiss 7.5mm, weighed in the balance.

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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 reminds 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 Mister 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.

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As usual, the measuring arsenal 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, drip type coffee maker. 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!).

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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.

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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 crimping of the case into the bullet cannelure.
Many precision shooters swear by a crimp to encourage consistent powder burn.

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Two questions came popping into mind while looking at this wax...
(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 (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 is going to 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 load bench. It’s so uniform that no time was wasted making
graphs. If the reader wants flat lines..... check out the average politicians
brain scan.

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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 variation seems 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.

Tearing down the ten round sample, we find this:

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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 article!). 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 grain size it looks like AA2015br powder, but has the same shiny
coating 'look' that IMR stick powders have.

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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 in the flat base 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!

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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.

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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!
Proud alumni of Transylvanian Polygnostic University. "Know enough to be afraid."

"Vertroue in God en die Mauser".-Faith in God and the Mauser.

"Send lawyers, guns and money." -Warren Zevon

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Re: A look inside: GP-11 7.5x55 Swiss

#2 Post by samsplacci » Mon Jun 12, 2017 2:27 pm

Reinforces the findings by many of us who shoot the GP 11 regularly via a variety of Swiss Rifles. In my opinion, it is even better quality than most factory loaded ammunition.

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Re: A look inside: GP-11 7.5x55 Swiss

#3 Post by Rapidrob » Sun Nov 12, 2017 8:33 pm

Since the original posting of this article is long gone, the statement about the wax ring has an answer. The wax ring is vaporized when the cartridge is fired and coats the bore. This will do several things. It lubes the bore to help the steel jacketed bullet traverse the rifles bore with less friction. It keeps the powder fouling softer than a dry bore. This helps reduce corrosion. The grease aids in the cleaning the bore after firing. Since fired cases are dry to the touch once fired the wax may not help with extraction. It you shoot the cartridge with a piece of cardboard closely off to one side of the muzzle,there is wax on the cardboard.
On the old SRF we did an experiment mixing the wax and loading the case with a similar weight and type match bullet. A wax "ring" was made by using a heated wax mix and a small needle syringe. It did work, but unneeded due to our bullet jacket material. Plus there was so much surplus 7.5 GP11 ammo for sale at the time it was time consuming.
I know of no other ammo loaded for a military that is as consistent or of such a high quality as Swiss GP11.
I have not shot,nor know anyone who has shot the "new" made GP11 replacement ammo in 7.5 Swiss.
Tin Can Sailor
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NRA Endowment Member
President New Mexico Military Surplus Rifle Pistol Shooters

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