Leading in one barrel and not the other

[bruce drake]

Folks,

I've just recently completed building my 7.62x39 Mauser and I took it and my Yugo Model 59 SKS (the one without a grenade launcher on the end) to the range today to test fire some cast loads with them. Everything was fired at 50 yards for testing purposes.

Here were my loads for the day:

169gr 311413 GC
Sized .312
Lubed with Carnauba Red
20gr AA1680

170gr 311291 GC
Sized .312
Lubed with Carnauba Red
20gr AA1680

160gr GC Lee Tumble Lube
Sized .311
Lubed with Lyman 50/50
19gr AA1680

The new Mauser has a 22" barrel and put all loads into 2" groups. Of course each had its own Point of Impact. I definitely am looking forward to using this rifle in the future. The barrel had a beautiful lube star after 20 rounds and had some carbon fouling to deal with.

The SKS (18" barrel) functioned with all loads and kept them within an average of 4 inches. Problem with this rifle was at the end of about 30 shots, I literally flaked the lead out of the muzzle crown with my finger.

The only difference in the two was that I fired the last ten (the Lee 160gr) in the SKS at a faster rate of fire than my Mauser.

Now is this an issue of the new barrel (.309) diameter versus a military surplus barrel (.311) or do you think the lube failed because of the high rate of fire?

I may size the Mauser bullets in the future to .310 to try for a smaller group... I just want to develop an acccurate load to work in both rifles (Semi-Auto function in the SKS would be preferred).


Bruce

[GrizzLeeBear]

Don't take any of this wrong, I don't know how much experience you have shooting cast boolits, but did you actually slug the bore of the sks as being .311 or just stating the nominal size? If not you need to slug it to find the actual size, from what I understand sks barrels can vary quite a bit in their bore sizes. Why were the 160 gr. bullets sized .311 and the others .312? Even if your barrel is .311 shooting .311 bullets at what 1700 - 1800 fps? might very well cause leading. If the barrel is over .311 in would lead for sure. What ever it slugs at size to .001 to .002 bigger and see if that doesn't help.
I don't think the rate of fire was the problem for only 30 rounds. I shot a highpower rifle match with my Mauser last weekend and although its not a semi-auto, we fire 2 strings of 10 in 60 - 70 seconds in the sitting and prone rapid fire stages (plus it was like 95 degrees) so they get plenty hot. My barrel shows very little or no leading. I would guess its more of a bullet - bore fit issue.

[bruce drake]

GLBear,

The Yugo SKS bore was unissued when I bought it two years ago. I slugged it with a soft fishing sinker and wrote it down as .311 on the grooves and .309 on the lands.

The reason the 160gr bullets are .311 was because the Lee mold drops them at .311 while my Lyman molds with the same alloy drops them at .313 and .312 respectively.

I also shoot highpower matches with cast bullets quite regularly. In fact my intentions on the 7.62x39 Mauser is to build it into a Reduced course rifle for my son when he turns 12 in a few years.

My main concern was regarding the change in lube from one type to another. I've always liked the Carnauba but the 160's with 50/50 were a leftover from another experiment.

Bruce

[versifier]

Bruce,
I think that what is causing the mil barrel to lead is the way it was drilled/reamed/rifled. Truly is it written that every barrel is a universe unto itself. I betcha if you looked into it with a borescope it would look like the lunar surface, typical of barrels from that place and time. Now, it might be the lube, but frankly I doubt it, or you would have observed "lube failure" in both barrels from driving the boolits too fast for the lube/alloy combo, but you didn't. I believe it was the greater friction between boolit and barrel in the relatively poorly finished barrel. There was a reason why so many SKS's & AK's had chrome lined barrels - it hid a multitude of machining sins, (and most, if not all, were made after they stopped making/using corrosive primers).

[bruce drake]

That might be so. The Yugo 59 SKS does not have a chromed bore. The Yugo 59/66 does and is probably be a reason for the difference.

I don't have a borescope here at the house but I might bring the one from work home overnight ot look at some other barrels of mine.

I don't want to consider the idea of using J-words with the SKS just yet but in the process of getting the lead out will require me to reseason the bore with lube before I can consider shooting it with cast again.


Bruce

[9.3X62AL]

Bruce--

.311" bore is well within the range of normal, but .309" across the lands doesn't sound right--land height in most nominal 30/31 calibers is around .004", so I would expect a land (bore) diameter more like .303". I would re-visit that slugging project again......but if your initial findings are correct, a .001" land height would likely have trouble maintaining a grip on a casting.

[GrizzLeeBear]

Bruce, maybe you could shoot some of the Lyman bullets as-cast through it since they are dropping at .312 - .313. Just run them through a .313 sizer to seat the check and lube. If it still leads I think Versifier might be right, just a rough bore. Maybe do some hand / fire lapping?
I don't really believe the whole "seasoning the bore" thing, well not for most rifles anyway. After shooting some J-bullets in my Mauser, I just soaked it good with Sweets 7.62, let it set for 5 - 10 minutes, run few patches, then bronze brush about a dozen strokes, then patches till they start comming out clean. This work pretty well to get all the copper out. Then went back to shooting cast boolets and they go right where they're supposed to.
But then again, like they say, every gun is a law unto itself.
BTW, what kind of groups you get at 100 with the Yugo SKS?

[bruce drake]

GLBear,

I haven't had a chance to scrub out the rifles yet to slug it again but the SKS shot an average of 4-5" groups at 100 with Wolf 123gr and Olympic 125gr Ammo (The Olympic Ammo is Boxer SR Primed!)

Bruce