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Posted: 12/11/2011 4:23:54 PM EDT
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i recently bought a spanish mauser model FR8 rifle and will reload for it.
a little history before i begin. the FR8 is based on a spanish mauser 98 action. it's a bolt action that was re-barreled from the 7.92x57 cartridge to the 7.62x51 NATO cartridge, basically to be used as a training rifle for the military and police use. it's an odd duck but looks cool. the rear of the gun looks like a typical mauser bolt action, but the barrel at 18.5 in. has an AR style flash hider and what looks like a gas tube below it. it's actually a place to mount a bayonet and inside there is a storage place for a cleaning kit. back on track. it's known that the spanish mausers of that era (built during WWII) have a softer steel than the german mausers. therefore, it's advised NOT to shoot modern .308 ammo through the rifle due to the higher pressures. all the military surplus / reloading sites agree on this. the 7.62 NATO ammo is supposed to be within safe parameters. so, using the hornady 6th edition reloading manual as a guide i set about making handloaded ammo. i am using new winchester .308 brass, CCI 200 L/R primers, IMR 4895 powder and hornady 168 gr. A-MAX bullets. oddly, the section for 7.62 NATO section starts out with 36.0 gr. of powder vs. 35.0 gr of powder for .308 as a starting load. as the .308 powder charges were more conservative i went with that. at 35.0 gr. i noticed that the primers were backed out of the fired cases. i then increased the powder charges by .5 gr. increments. the last batch i shot at 38.0 gr. had only a few slightly backed out primers. these rounds chronographed at 2150 fps. it's my understanding that low pressure will cause the primers to back out of the case. is that correct? the max load for a 168 gr. bullet with IMR 4895 powder is 43.3 gr. (again from the hornady book). should i gradually increase the powder charge until the primers are no longer backed out? at this point the fired cases look perfectly normal with no high pressure signs and the primers are not flattened, just backed out of the case. by the way, my rifle was never issued out for use and was in unfired condition. i don't want to damage the rifle or myself by loading too hot. any advice will be greatly appreciated |
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Yes, low pressure loads can cause the primer to back out, so can excessive headspace. As stated above, have it checked out.
The 7.62 does not run at a lower pressure than the 308 win. They are identical. The 308 runs at 62Kpsi and the 7.62 runs at 50K CUP. 50K CUP equates to 62k psi. Here is a little observation to help clarify. Fed Ammunition 7.62x51 168g Match Muzzle Velocity=2650fps http://www.midwayusa.com/product/953713383/federal-american-eagle-ammunition-762x51mm-nato-168-grain-open-tip-match Fed Ammunition 308 Win 168gr Match Muzzle Velocity=2650fps http://www.midwayusa.com/product/953358610/federal-premium-gold-medal-ammunition-308-winchester-168-grain-sierra-matchking-hollow-point-boat-tail The only way these two rounds can have equal velocity is if they are running at equal pressures. Take a look at this document, check out the load data. This is for the 7.62 Nato round. It lists a charge of 44gr IMR 4895 with a 174gr bullet. It also lists velocity of 2640 fps. Now go through all of your 308 Win load data and find a load that compares to this military 7.62 load. What you will find is that the Military load 44gr of IMR 4895 is hotter (higher pressure) than most 308 Win rounds. http://www.ar15.com/media/mediaFiles/348/32231.JPG If you look through military ammo specs from before the mid-90's, you find military copper crusher numbers reported as "psi" at the same time a commercial maker would have reported them as "CUP" (I don't know why "CUP" gets caps while "psi" gets lower case, but that's how SAAMI publishes them). The SAAMI conformal piezo spec is 62,000 psi MAP (Maximum Average Pressure). I'll have to check, but I recall SAAMI MAP allows up to 4% deviation from average, so 64,480 psi would be the allowed extreme spread in SAAMI compliant ammo. The NATO allies use 415 MPa, which the CIP uses for both 7.62 NATO and .308 Winchester. That converts to 60,191 psi, but the CIP measuring system sets the sample point further forward in the case, so it sees a bit of the pressure drop at the bullet base that occurs when the bullet starts moving. That makes them tend to read about 2,000 psi lower than the SAAMI setup does in this pressure range. So the same ammo would measure closer to 62,000 psi in SAAMI equipment. |
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Here is a link to the SAAMI pressure ratings for Rifle cartridges. It list the 308 Win in both CUP and psi. 52K CUP and 62K psi.
http://www.saami.org/specifications_and_information/publications/download/206.pdf |
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Up the charge weight, 38 grains of IMR 4895 with a 168 grain MK is way too low.
You should be able to load 41.5 grains without a problem. That is still a mild load. If the primers keep backing out, then have the headspace checked. In the mean time, since it's obvious that nominal ammunition chambers, put a piece of scotch tape on a case head, trim it so it doesn't hand over the rim, and try to chamber the cartridge. The bolt handle should not close, and the gun must be checked out if it does. The tape will be about 0.002 inches thick. I'm no fan of FR-8's, so that's about all I have to offer. (They are the hardest natural kicking rifle I think I've ever fired.) On a peripheral topic, I don't want to see this thread get off topic on the discussion of .308 Win and 7.62X51 chamber pressure. Start a new thread for that discussion. |
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excellent feedback gentlemen, thank you.
tomorrow when i head out i'll stop by my local gunsmith and have him check the headspace with his gauges. i'm thinking it should headspace o.k. as it never was fired other than proof testing, but i won't leave it to chance. the next batch to shoot are going to be 38.5 and 39.0 gr. loads. from what you have showed me they are probably also going to be too light. there's still plenty of leeway to move up to stouter loads safely. and yes AeroE, the recoil of the FR8 packs a wallop  .
great fun though  .
thanks for a well run and informative site. |
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Quoted:
tomorrow when i head out i'll stop by my local gunsmith and have him check the headspace with his gauges. i'm thinking it should headspace o.k. as it never was fired other than proof testing, but i won't leave it to chance. Get a Hornady Lock'n'Load gage. Measure the headspace of your fired cases, then the length of your resized cases. The fired cases should measure within acceptable limits for the spec on 308. The resized cases should be about 0.002" to 0.003" shorter than your fired brass. Any more than this is just too much. I would bet that about half the problems on this board could be solved thru the use of this set of gages. |
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