Finnish M39 & D166 bullet

[shil]

Hello, Everyone. I'd like to ask for some help. I have a Finnish M39 Mosin-Nagant for which, in the interest of authenticity, I am loading the Lapua D166 200 grain bullet to replicate the military loading. The bullet is a .310. I'm using Lapua cases. Sizing the neck to .310 seemed to yield a loose bullet when seated to the OAL in the Lapua/Vihtavuori manual unless heavily crimped. I tried sizing to .308 for more bullet pull and was successful with loading, but I'm getting gas blowback. The case necks and walls get sooty, and sometimes I can feel the gas (always wear shooting glasses!). I measured the inside diameter of one of the fired cases and got .308, meaning to me that the case neck probably did not expand to seal the chamber, hence the blowback! As I recall, I didn't have any problem with Prvi factory loads or Hornady 174 grain .311 hand-loads with Prvi brass. Has anyone else encountered this, and, can anyone advise me? I've been hand-loading for 30 years and this is the first time I've ever experienced anything like this.

Gene

[Unclenick]

Welcome to the forum.

You have a classic low pressure sign and need a bit more powder. What are your load details (powder charge, primer, cartridge overall length)? If you can weigh a fired case and fill it with water and weigh it again, that will make a rough pressure calculation possible.

It is normally required that you neck at least 0.0005" under bullet diameter, with 0.001"-0.003" under being more typical. You need the neck ID to get undersize. If you get a bushing die, you can buy a neck bushing the exact size you need.

[shil]

Thanks, Unclenick. I appreciate your help, and it's nice to be here on the forum. It appears I can learn much. Even at nearly sixty-one years of age, one can never stop learning. The load is out of a Vihtavuori manual. I'm using 40.7 grains of Vihtavuori N150 powder with a CCI200 primer and Lapua brass. The load is shown as maximum.

[shil]

Oh, sorry.......... OAL was by the manual, 2.992 inches.

[Unclenick]

And the water capacity? Weigh a fired case with the primer still in it, fill it with water level to the top but with no meniscus. Make sure the bubbles are tapped out and the outside is wiped dry if any overflowed, then weigh again. The difference in the two numbers is what I am after. It's called case overflow water capacity, even though you don't literally overflow it. It's just a way to figure out the actual volume inside the case after firing.

Oh, and just to make sure, your's is chambered in the Finnish 7.62×53, and not 7.62×54, right?

[Unclenick]

I just did a quick look at your bullet. I'll bet this is your problem: you've got the Finnish rifle with 0.310" bore, but Lapua makes the D166 in both .308" and .310" to cover both the earlier MN's and the M39, whose bore was made bigger to accommodate Russian ammunition. If you've purchased the .308" version of the bullet, that would explain the die problem and the low pressure, both. Unfortunately, the .308" probably won't shoot great in a bore that loose. You'd need a warmer load pressure to bump it up.

I think your next step, if you don't already have one, is to get an OD thimble micrometer both to check your bullets and to slug your bore. This one will work fine for this purpose, and for $1 more, this one has a mechanical digital counter for the more significant digits and a vernier for ten-thousanths. You do have to know how to read a vernier scale with these unless you want to spend over four times as much to go electronic digital. Unfortunately calipers are just not accurate enough for bullet and bore slug diameters, even though some try to get away with using them.

To slug the bore you need a soft lead bullet or ball. A swaged .32 wadcutter may work, but a cast one will be too hard for accurate slugging. A Hornady .36 cal round lead ball will work. If you can find plain lead conical or egg-shaped fishing sinkers, they will work, but a lot of the ones I see these days are something harder than pure lead. Anyway, clean the bore and run an oily patch down it to leave a very light coat of oil on the bore. Use a plastic head hammer or a short length of wood dowel with a steel hammer, and tap the slug into the bore at the muzzle. You may shave some extra lead off, but that's OK. Then push it slowly through the bore with your cleaning rod. This lets you feel for tight and loose spots. Measure the slug across the slug with the micrometer. This will tell you what size bullet it actually wants. Add a thousandth to that for cast bullets (this rifle should make a good cast bullet shooter if the muzzle hasn't been funneled by excessive cleaning rod rubbing.

P.S., I noticed several comments that the 174 grain, 0.311" diameter Sierra MatchKing shoots especially well in these rifles. If you want to learn its accuracy limits, that should do it.