Reloading Experiences
I have two K31 rifles, and both barrels and chambers will accept .308 diameter bullets. With these foreign rifles of unknown chamber dimensions, it is a prudent safety check to verify the neck expansion on a fired case mouth. A bullet should enter the expanded case mouth without resistance. If not, the case neck is pinching the bullet during ignition and pressures may be dangerous.
Cartridge OAL
American reloading guides provide inconsistent information on the 7.5 * 55 Swiss cartridge. I suspect all their data is based on reverse engineering from the older, M1896 and M1911 rifles. The K31 has been on the market for only a short time. I found , as many others have found, the maximum cartridge lengths of 3.060”, listed for the 168 Match Bullet in the third edition Hornady Manual were much too long. When I loaded the US military .308 174 gr FMJBT bullet anywhere near this length, the bullet would jam in the barrel throat. I finally found an OAL of less than 2.900” work in my rifles. I have standardized on an OAL of 2.850 with nothing greater than 2.900” and have had no problems. I have used this for 165 grain hunting bullets, 168 grain Sierra and Nosler Match bullets.
I purchased INDEP 7.5 Swiss, Lot 9001, loaded with a 170 FMJBT bullet. These cartridges from the factory have an OAL just below 2.900”, the spread between five I measured went from 2.880” to 2.894”.
Trim Length.
The manuals state that the maximum case length is 2.180”. The instructions that come with Lee Dies state that maximum trim length is 2.185”. My third edition Hornady Manual gives a 2.140” case length. From unfired INDEP ammo, the case length of 20 cases, the spread was between 2.167” and 2.1765”. Most of the cases were just about 2.170”. I believe that any trim length between 2.180” and 2.160” will work.
It is important to trim your ammunition on the first reload. I have found that cases grow the most on the first reload, and become over length. The consequence of over length cases is that the case neck will extend into the tapered throat area, pinching the bullet. Excessive pressures will develop because of this obstruction. This can lead to blown primers and/or sticky extraction.
Brass
7.5 * 55 Norma brass and loaded ammo is ridiculously expensive for this rifle. However, 284 Winchester brass can be sized, necked up and fireformed for use in this rifle. Even though the OAL of a 284 Win case is supposed to be 2.165”, I found that the sizing process made them overlength and cases needed trimming. 284 Winchester brass has same case rim diameter as the 30-06, which is smaller than the Swiss round. I measured three 284 Win case rims and found them to be 0.427”, 0.470”, and 0.470”. Three Norma case rims were 0.498”, 0.497”, and 0.495”. This dimensional difference has caused occasional failures to eject. The cartridge will extract out of the chamber, but will fall off the bolt face into the action. I encountered no feeding problems with 284 brass. The case capacities of my 284 Win brass are less than the Norma 7.5 cases. Behind a 174 gr FMJBT it took 55 grs of WC852 in the 284 brass to give the same velocity as 57 grs of the same powder in the Norma cases.
INDEP ammunition has been available on the US Market, and a measurement of five cases gave a case rim measurement spread between .4935” and .494”. All the INDEP cases ejected positively due to this big rim. The weight spread, for twenty INDEP cases, was between 180.0 grains to 184.1 grains. My Norma and 284 Winchester brass is all loaded up, so I cannot make a weight comparison between them and INDEP brass.
Bullets
Different manuals give different bullet weights. I have found in reloading for military rifles that the best results come from loads that duplicate as close as possible the standard military loads. This makes sense if you think of it. Both the rifle and the ammunition were developed together, and then over the years, military rifle acceptance tests were conducted with the military ammo, and the military ammunition makers tested their ammo in the military rifles. So, in time, the rifle and ammunition evolved together to give optimum performance.
Standard .308 diameter bullets have worked well in my rifles. The fourth edition of “Cartridges of the World”, by Frank Barnes claims that a 174 grain .308 bullet was adapted in 1911. The web site
www.swissrifles.com has more current information, that is “The GP11 was loaded with a 174 grain spitzer bullet. The round could travel at 2640 fps. The diameter of the bullet was increased to .307 inches, and the length of the case was increased to 55m. The GP11 generates around 45,500 psi of pressure.” The site provides the warning “ Under no circumstances should GP11 rounds be fired in Model 1889 Schmidt-Rubins!! “
In my rifles, point of impact, with bullets from 165 grain to 174 grain, shoot to the mechanical point of aim.
Reloading Dies, and Sizing.
I am using the Lee Sizing Die and Lee bullet seater. The table reflects my load development. I followed the load data included with the Lee Die, and found it unsatisfactory. The loads are too slow for the K31, only the IMR 4895 load was close to an acceptable velocity. And it happened that the WC852 I purchased from Jeff Bartlett,
http://www.GIBrass.com/, turned out to be a champ with this cartridge. WC852 is military surplus 30-06 ball powder. Accurate Arms purchased a different 90,000 lb lot of this, repackaged it, and sold it as 2700. 2700 was the same basic powder but had a faster burning rate than my lot of WC852. Since I found a very satisfactory load with WC852, I stopped experimenting with other powder combinations. But, if I get tempted to start experimenting again, I expect the powders that have a good probability of providing good results to be IMR 4895, IMR 4064, and IMR 4350. The velocities that I want to get to will be just below 2600 fps with a 168 or 174 grain bullet. My groups definitely tightened up as bullet velocities approached this speed.
The Indep load, that I recently tested, actually shot better than I thought it would. It was Lot number 9001, and purchased from SOG. Cases only had 33.1 or so grains of an unknown ball powder. The case was less than half filled with this powder, and ball powders usually do not work well under 80% loading conditions. The 170 grain FMJBT bullet was .3075” in diameter, except at the boatail edge, where it was .308” in diameter. When I pulled the FMJBT bullet, and substituted a 168 Nolser bullet, I got a better group. Unfortunately the Indep factory load is a low powdered load, giving an average of 2155 fps, and my rifle shoots best with faster loads.
Temperature effects velocity, and I believe the loads that I chronographed at 52 F would be at least 100 feet per second faster at 70 F.
My rifle was sensitive to bullet velocity. The slower 2100 fps loads, printed about 2” low and to the right of the 2500 fps.
Code:
[SIZE="3"]7.5 X 55 Swiss
Load Velocity
FPS Ambient Group Size
174gr .308 FMJBT 57.0 gr WC852
Norma cases, Fed210S OAL 2.900” 2563 T =70 F 0.75*1.3”
174 .308 FMJBT 46.0 gr H4350, Norma cases, Fed210S OAL 2.925” 2146 T = 70F maybe 3"
174 .308 FMJBT 42.0 gr IMR4895, Norma cases, Fed210S, OAL 2.925” 2429 T = 70F Maybe 3”
165 gr BTSP Hornady 55.0 gr WC852, 284 Win Brass, Fed 210S OAL 2.900” 2595 T = 90F 3.0"*1.6"
174 gr FMJBT 55.0 gr WC852, .284 Win Brass, Fed210S, OAL 2.900” 2531 T = 75F About 2”
168 Nosler 56.0 gr WC852 INDEP Brass and Primers, OAL 2.850” 2404 T = 52 F 2.8” * 1.35”
168 Nosler over INDEP Powder, brass, primer, OAL 2.850” 2112 T = 52 F 1.8 * 1.9”
170 gr FMJBT Indep factory 2155 T = 52F 3.6*2.1[/SIZE]”
Cartridge Comments:
Loaded 7.5 * 55 Norma cartridges boxes claim 2650 fps with a 180 grain bullet. Any rifle/cartridge combination that can send a 180 grain bullet to 2650 fps should be considered high power. I believe the basic case design goes back to 1889. The current cartridge configuration seems to have been set in 1911. It has some desirable characteristics for automatic gun mechanisms: It is shorter than the 30-06 and has a very thick rim. A compact action with less mechanism travel can be designed around a short cartridge while a thick rim is harder to pull off during extraction. These are characteristics that drove the replacement of the 30-06 with the 308 Winchester as a military round. I wonder if the 7.5 * 55 would still be in our military inventory if the US had copied it instead of creating the venerable 30-06.
One weekend turned out to be warm enough to go out and chronograph some loads. My Chrony Chronograph does not do well in forty degree weather, which is normal for this time of year, but we got a 58 F weekend, and so I was able to do load development with my chronograph.
I have a keg of AA4350 and was curious to see how it does in the 7.5 Swiss. Based on case capacity, the 4350 series looked like it should give good results with 168 grain and up bullets in the 7.5 Swiss. AA4350 is advertised to be equivalent to IMR 4350. I have found that to be in the main true, accepting the differences you normally have with lot variations. That is, you have to be very careful with maximum loads if you change to a different lot of powder, even though it is the same powder by the same manufacturer. Accurate Arms has told me that they keep lot to lot variances within 5%, and claim that is half the industry standard. If that is true, then it is possible to change powder lots, and have a pressure increase of 5000 psi for a load that normally produces 50,000 psi. Then of course, a max load that is safe in 58 F weather may be too hot in 90F weather. These are things you just have to check out, unless you own a pressure barrel and a thermal chamber.
Anyway I looked at my Sierra Loading Handbook for load data and found that Sierra had an extensive section for the 7.5 Swiss. Sierra had tested their loads in several M1911 rifles. They claim a maximum load of 51.0 grains of IMR4350 with their 168 Match Bullet. The velocity they publish is 2700 fps out of a 29” barrel.
Since the K31 is a much stiffer action , it is a front locking action whereas the M1911 is a rear locker, I was not afraid to use their maximum load, and than go one grain above it to see what happened. Also, I used the 168 grain Nosler Match Bullet, which is an exact copy of the Sierra, except perhaps, jacket thickness. Glen Zediker (
http://www.zediker.com/) claims in his book “Handloading For Competition” that the J4 jackets are thinner than the Sierra.
I have found that the 168 grain Sierra Match bullet is one of the most consistently accurate bullets in .308 caliber rifles. It is a truism that if you own a 308 Winchester and your rifle will not shoot a 168 grain Sierra Match, in front of 40.5 grains of IMR 4895, then something is seriously wrong with the rifle. I have had excellent results with the less expensive Nosler copy of the Sierria bullet in 308 and 30-06, and I have been using it in my 7.5 Swiss with good results.
Anyway for a load of 168 grain Nosler, 51.0 grains AA4350, INDEP Brass, Fed 210S Primers, OAL 2.850”. I measured an Average Velocity of 2536 fps, with a standard deviation of 17fps. Accuracy was within two inches at 100 yards with iron sights. For 168 grain Nosler, 52.0 AA4350, INDEP brass and Federal Primers , OAL 2.850”, I got a velocity of 2632 fps with a standard deviation of 24 fps. Accuracy was good, five shots falling within .075” * 1.5”. Considering that all groups were shot with iron sights, I do not believe that there is any significant difference in accuracy between the two loads. Extraction was easy on both loads, and primers were rounded. When doing load develop with a bolt gun, if the rifle design allows it, I will pull the cocking piece back and then open the action. This gives me a more sensitive feel to extraction resistance. If it feels like you are extracting a cork from a bottle, your pressures are way too high. I miked the case heads, comparing them against unfired brass, and was unable to determine any conclusive patterns. Everything was within .001” of unfired brass, and I do not know if that is significant or not. I have never had any luck determining pressures from case head measurements though such authorities as Ken Waters use it in load development.
I think the 51.0 grains of AA4350 may turn out to be a pretty good load. The velocities spread was fairly even and it shot to exactly to point of aim at 100 yards with the rear sight set at its 100 meter mark. A six O’Clock hold gave a group exactly at 6 O’clock in the bull. When it gets warm again, I may try some IMR 4064 loads with the 168 grain Bullet.
Accuracy:
While I have shot groups that were less than 1 MOA I consider this to be just a statistical aberration. While the K31 Swiss rifles are exceptionally accurate for a service rifle, but, in the end, they are service rifles. I don't know the Swiss Army requirements for accuracy, but no one builds a general purpose service rifle with a 1 MOA requirement. (Well, I don't know of one) Typical bolt gun era service rifle requirements were around 3 MOA. That was way inside the hold of the untrained cannon fodder sent to the fronts. Accuracy requirements were even worse for gas guns. The M14 was a 5 MOA affair, as issued. I don't know if an SKS will hold 8 MOA. It is possible the Swiss wanted 2 MOA rifles but no one should expect consistent true target rifle accuracy out of one of these things. What they can expect is outstanding accuracy for a service rifle.
Incidentally, the Swiss greased their bullets up to the 1980's. They claimed better accuracy and a longer barrel life. I don't have any GP1890/03 rounds, but from pictures, it looks they were greasing bullets all the way back to the 1890's.