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Mountain Molds • 357 Maximum rifle
Page 1 of 1

357 Maximum rifle

Posted: Sun Mar 08, 2015 7:36 pm
by mtngun
My 357 mag Green Mountain TC barrel had a sloppy (0.363" - 0.364") throat, for reasons I never figured out. Rather than setting the barrel back to rechamber it, I thought I'd try punching it out to 357 Maximum. The maxi happened to be just long enough to clean up the sloppy throat.

This time I made the throating reamer seperate from the chambering reamer, so I could tweak the throat independently of the chamber. The throat has 1/2 degree taper per side. While the reamer does include a short freebore section, for now I only stuck it in deep enough to cut the tapered lead, but not the freebore. The taper begins immediately after the case mouth at about 0.3585".

I'm not sure whether freebore is good and necessary for cast bullets, but one thing I do know is that I can always re-cut a throat fatter and longer, but I can't re-cut it skinnier and shorter. :lol: So I am starting out with no freebore, and keeping the option to add freebore later.

Homemade reamers. The dimensions turned out well and they cut well. There were no unpleasant surprises. 8-)
Image

OK, so now the barrel has a PERFECT throat. It should be a tackdriver, right? I loaded up some of the 160 gr. plain base spitzers that had been the best performers in the 357 mag, except this time I sized them down to 0.359" rather than 0.360", and seated them about 0.015" off the rifling for easy chambering, which worked out to be 1.994" COL. I walked the charge of WW296 up until I encountered sticky extraction, then walked the charge back until they extracted easily, which turned out to be 20.6 grains. I used Federal 200 primers because these cases happened to be already primed with them, leftover from a late 80's project. It so happened that the bullet could not be crimped at that COL.
Image

So how did the PERFECT throat shoot? Not so great. There was some light leading in the barrel -- mostly in the middle -- and accuracy was worse than the sloppy throated magnum. :(

-- 4 MOA for 10 shots at 100 yards (vs 1.5" for the magnum)
-- 1.29" mean radius (vs. 0.55" for the magnum)
-- 1.69" radial standard deviation
-- 2167 fps (vs. 2000 for the magnum)
-- 0.98% standard deviation (vs. 0.8% for the magnum)
Image

Why Does The PERFECT Throat Shoot Worse Than the Sloppy Throat?
-- the plain base bullet may be crapping out at the higher velocity.
-- the bullet may be shaving lead in the PERFECT throat.
-- the 0.359" bullets may be too big for the PERFECT throat.

The reason it may be shaving lead is because I am cramming a 0.359" bullet into throat that may be only 0.3585" or so (I haven't made a throat impression yet, only some quick checks with pin gages). There is a 45 degree chamfer between the chamber and the entrance to the throat, but it's possible that it's still too abrupt to avoid shaving lead.

Things To Try Next Time (or the time after that)
-- reduce the velocity to 2000 fps, same as the magnum. If it still leads the barrel and still groups poorly, then we'll know the root problem is the PERFECT throat, not the higher velocity.
-- try 0.358" diameter.
-- if neither of those things makes it perk up, then I'll probably conclude that this 160 plain base does not like the PERFECT THROAT.
-- At which point there will be other things to try -- gas check, or tweaking the throat to make it even more PERFECT. :lol: Also, arranging for a crimp groove, and optimizing the COL. But for now my main concern is figuring out if the throat is optimal.

Re: 357 Maximum rifle

Posted: Thu Mar 12, 2015 4:17 pm
by mtngun
Update:
-- Today I attempted to make a throat impression, but the alloy I used was too hard (someone had given me some alloy that they claimed was 1% tin, but after trying to use it to make a throat impression I'm guessing it is closer to 10% tin :lol: ). For what it is worth the not-so-great impression showed 0.3583" - 0.3585" immediately in front of the case mouth, tapering down to about 0.357" over 0.200".

-- I sized some bullets 0.358" (they actually came out 0.3577" or so) and was able to push one about 0.139" into the throat, using the eraser end of a pencil as a pusher.

-- I sized some bullets 0.357" (they actually came out 0.3567" or so) and was able to push one about 0.200" into the throat, using the eraser end of a pencil as a pusher.

-- I remembered that out of the magnum chamber, and they had not fouled the barrel.

-- In hindsight, the 0.359" bullets that I shot last time were too fat to enter the 0.3585" throat. They probably shaved lead as they entered the throat, and then they had to jump through about 0.200" of freebore before they contacted the rifling. It's no surprise they shot poorly.

Lessons Learned and Stuff to Try Next Time
-- make a good throat impression using pure lead

-- try the 0.357" and 0.358" bullets, seated into the throat as far as possible.

And then we'll go from there. :)

Re: 357 Maximum rifle

Posted: Sat Mar 14, 2015 6:01 pm
by mtngun
This experiment employed 0.3568" and 0.3577" bullets seated out into the throat as far as practical. The theory was that if the poor accuracy and leading with the 0.359" bullet was due to the bullet shaving as it entered the 0.3585" throat, then these bullets would be less likely to shave lead.

The 0.3568" bullet. 160 gr. plain base spitzer, 22.8 gr. WW296, CCI450 primer, oven treated WW, HVR lube. A light crimp into a lube groove. 2.186" COL.
-- 2143 fps
-- 4.25% velocity standard deviation for 20 shots. That's HORRIBLE. :o
-- 10 shots groups at 100 yards 5.44", 10". :cry:
-- the barrel fouled from the very first shot (which was only 2051 fps). It wasn't severe fouling, but it was enough that I could see it with the naked eye, and cleaning the barrel afterwards produced visible specs of lead on the patch.
-- the intent was to adjust the COL so the front band was 0.015" away from making contact. Nonetheless, some rounds were difficult to chamber. A couple of empty cases had significant lube buildup on the neck, making me wonder if lube buildup in the throat and chamber were responsible for the inconsistent chambering ???

Note that while the first group (right) was poor, the 2nd group (left) was totally wild, a symptom of fouling.
Image

The 0.3577" bullet. Same load as above except a shorter 2.125" COL. In response to seeing lube buildup on fired cases with the 0.3568" bullets, this time, after seating the bullet, I took a rag dampened with WD40 and wiped any excess lube off the exposed part of the bullet. There was still lube in the grooves, just not anywhere else. And none of the fired cases showed lube buildup.
-- 2226 fps
-- 1.88% velocity standard deviation for 20 shots. That's poor, but only half as poor as the 0.3568" bullets.
-- 10 shot groups at 100 yards 4.38", 6.25". Note that the 2nd group was worse, probably due to fouling.
-- the barrel fouled enough that I could see it with the naked eye.
-- once again, chambering was intended to be easy, but some rounds required effort to chamber, and 4 rounds popped the action open, which can be a sign of the action not locking up 100%.

The 3577" load.
Image

Throat impression. I used a shortened case so the impression would pick up part of the chamber neck. No sign of burrs or gouges. No sign of the chamber being off center. Dimensions are practically identical to the reamer dimensions, so it appears that the homemade reamers cut perfectly. If there is a problem with the throat, it is a design problem, not a machining problem. ;)
Image

Re: 357 Maximum rifle

Posted: Sat Mar 14, 2015 7:07 pm
by mtngun
Conclusions .... or Confusions ???
-- I give myself credit for finally learning how to make reamers that cut well. 8-)
-- but my 1/2 degree per side throat, that was supposed to be the greatest cast bullet throat ever, is a failure.
-- the previous "bad" 0.363"+ throat shot better than expected with plain base bullets.
-- why would a sloppy throat shoot better than a "perfect" throat? :?:

I'll throw some thoughts out, but they're just "thoughts," not certain facts.
-- the bullet may still be shaving lead at the transition between the chamber and the throat. Maybe the "bad" 0.363"+ throat worked because the entrance was wide enough to swallow the bullet without shaving lead?
-- I believe (from memory) that the "bad" 0.363"+ throat was 1 degree per side.
-- the 1/2 degree per side angle makes for a very long throat. It's sort of like having long freebore, not necessarily a good thing for a cast bullet.
-- I believe the convention is to refer to the throat angle as degrees per side. So when a benchrest shooter says he has a 1 degree throat, he really means 1 degree per side, or 2 degrees included angle. Sometimes I get the convention mixed up, and call my 1/2 degree per side throat a 1 degree throat. I'll try to stick to the "per side" convention from now on.
-- CBA benchrest shooters generally use long bullets that can reach past a long throat to engage the rifling. My stubby 35 caliber bullets need a shorter throat if they are to reach the rifling.
-- CBA benchrest shooters use bolt actions to jam the bullet snugly into the throat, but that is a no-no for the Contender action. I've been shooting cast in Contenders for 30 years now, and have never had good luck with a load that required force to close.
-- I can always re-cut a throat with a more shallow angle, if I'm not happy with the original angle, but I can't recut a shallow angle to a steeper angle. Moral of story: next time try a steeper throat angle
-- so what is the perfect Contender cast bullet throat? I don't have an answer right now, other than to say that it needs to 1) chamber easily and 2) come close to the rifling but not actually jam the rifling.

Here is another guy's He suggests 3 degrees per side, though his application is different than mine.

My Contender barrel is currently 27" long, so I have enough extra barrel to experiment with several different throats to see what works and what doesn't work. It's frustrating when experiments don't work, but we learn as much from failures as from successes.
Image

Re: 357 Maximum rifle

Posted: Wed Mar 25, 2015 11:36 am
by mtngun
Autopsy of the "failed" 357 maxi throat. I wondered if perhaps there were a burr or sharp corner that was damaging the bullets? Or perhaps the throat was not concentric with the chamber or bore?

We're looking at the transition from the chamber to the throat.

No burrs or sharp edges are to be seen. There's no lack of concentricity that can be detected by eye.
Image
So ..... I have to assume that the 1/2 degree throat failed simply because it was a poor design. :cry:

I'm not totally convinced, but have nothing better to go on at the moment. :|