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6mm BR & switchbarrel bolt gun

Posted: Sat Feb 13, 2016 6:08 pm
by mtngun
This is what I have been working on lately.

M700 long action, Richard's Microfit stock, Jewell trigger. It'll probably get a 36x Sightron scope.

It'll be a nutless switchbarrel, more on that later.

Still lots of work to do to get all the barrels up and running.

Re: 6mm BR & switchbarrel bolt gun

Posted: Sun Feb 14, 2016 3:05 pm
by mtngun
The barrels have a hex milled at the muzzle so they can be screwed on and off without a barrel clamp. Some people don't like the appearance of the hex on the muzzle but this is a "work" gun so it doesn't need to win any beauty prizes.

After adding the scope & mounts, the weight is 16 pounds 1 ounce with a 1.25" diameter x 27" long barrel. :roll:

If this were for competition then I would have to use an expensive synthetic stock and a shorter, tapered barrel to "make weight." But synthetic stocks cost more money than the laminated stock, and tapered barrels cost more money than barrel blanks, and I'm not planning to use this gun for competition, only for load testing. If I ever decide to compete someday then I'll have an excuse to build yet another rifle. :lol:

In case you are wondering, the tape on the scope serves to keep things from rattling loose, which used to happen on a regular basis before I applied the tape. :lol:

The 6mm barrel is a 3-groove Pacnor that I found on ebay for $90. It may be a factory second because it seems like the groove diameter is a bit on the small side, but that won't matter for cast bullets.
-- 1.5 degree per side throat.
-- a short 0.245" diameter freebore
-- 0.270" neck
-- 0.242" groove as best I can measure
-- 0.2352" bore diameter

I was aiming for either a no-turn neck or a minimal-turn neck and it looks like I ended up with the latter. I haven't prepped cases yet so no shooting today. :(

UPDATE: I no longer put a hex on the muzzle. While I never had any problems with barrels that were screwed on hex-tight, for peace of mind I switched to taking the barreled action out of the stock and putting the barrel in a barrel vise so I could torque it. You can actually do this without removing the action from the stock but you need a roomy workbench which I don't have.

Re: 6mm BR & switchbarrel bolt gun

Posted: Thu Feb 18, 2016 2:39 pm
by mtngun
It remains to be seen which bullet diameter will be best for this tight 0.242" barrel. The smallest 6mm sizing die I had on hand was 0.245" so I made a 0.2435" bushing for my push-thru. If 0.2435" proves to be too small then it can always be honed out to 0.244".

Re: 6mm BR & switchbarrel bolt gun

Posted: Fri Feb 19, 2016 2:33 pm
by mtngun
Nose die, cut with the same throating reamer used on the 6mm BR barrel. Hence any nose sized in this nose die will fit the throat like a glove -- at least until the throat erodes, which doesn't take very long! :lol:

These nose dies are not difficult to make other than it can be a bit of a guessing game to get the depth just right. Ideally you want the entire bullet to be inside the die or nearly so. That way the entire bullet is supported and cannot bend or obturate (or at least it can't obturate bigger than the die dimensions). I may have cut this one too deep but I can prolly still make it work by using an extra long top punch. I won't know for sure until I assemble a dummy round with a sized bullet and see how it fits the rifle.

"A" section = 0.246" diameter
"B" section = 1.5 degree per side
"C" section = 0.236" diameter

Re: 6mm BR & switchbarrel bolt gun

Posted: Sat Feb 20, 2016 8:10 pm
by mtngun
Finally we had time to prep a few cases and actually shoot the rifle ! Yea ! 8-)

First I shot 3 firelapping bullets with #500 grit, hoping to knock off any machining burrs.

There were some 57 gr. GC bullets left over from the 6x45 project, sized 0.246" x 0.242" in a 3 degree nose die to match the 6x45 throat. I ran them through the 0.2435" push-thru so they ended up 0.2435" x 0.242". The nose will not be a perfect fit to the BR's 1.5 degree leade but we'll live with that for now.

I used Quickload to guess some powder charges that would produce around 2600 fps. At this time I have no fancy way to measure how far the bullet is away from the lands so I just seated them a little long and let the action finish seating the bullet when the round chambered. In other words, a slight jam fit.

-- 29 gr. WW 760. 2660 fps and a horrible 2.34% standard deviation. Not enough pressure (~21 ksi) to make this slow ball powder burn well.

-- 24 gr. WC844 (similar to H335). 2695 fps and 1.15% standard deviation.

-- 21 gr. Rx10. 2545 fps and 1.35% standard deviation.

As you can see, all three powders grouped about the same. We didn't break any world records but we did shoot consistent 2 MOA ten-shot groups at about 2600 fps. Not bad for a $90 barrel. :lol:

After shooting WC844 @ 2695 fps, there appeared to be some light copper fouling on the lands. From the gas check?

Taken at the end of the day, after shooting Rx10 @ 2545 fps. Very light leading near the muzzle. Prolly not enough to cause problems but I consider any fouling at all a bad thing.

Observations and Conclusions:
-- WC844 is the best powder so far. That's good because I bought 16 pounds of it and it didn't work worth a darn in the 6x45. :lol:
-- the 57 gr. GC seems to want to shoot 2 MOA come hell or high water. :lol:
-- the $90 PacNor is fouling more than the $140 6x45 Shilen. Not a lot of fouling, but not squeaky clean, either.

Things To Try Next Time and Down the Road:
-- try Rx10 at 2700 fps before I give up on it
-- there are 2 more powders I would like to try, WC845 and Leverevolution.
-- longer term, try different bullets, different diameters, different BHN, etc., the whole 9 yards. :lol:

Re: 6mm BR & switchbarrel bolt gun

Posted: Sun Feb 21, 2016 6:53 pm
by mtngun
I fired another 3 firelapping rounds, this time with #280 grit. Now continuing the powder testing with the 57 gr. GC spitzer .....

-- 25.7 gr. WC845. I believe this may be the military version of WW748. Or close, anyway.

-- 27.2 gr. Leverevolution

-- 22.2 gr. Reloader 10

-- 22.2 gr. Reloader 10 with a 46 gr. spitzer leftover from the 6x45 project.

15 shot groups because this rifle has been consistent enough that 15 shots is sufficient to give me a pretty good feel for how it is going to shoot. Not much difference in accuracy but there was a big difference in velocity variation, so that'll help me choose a powder for this project.

Velocity variation for all powders tested with the 57 gr. GC, from lowest to highest:
-- 0.9% Reloader 10 @ 2712 fps
-- 1.15% WC 844 @ 2695 fps
-- 1.35% Reloader 10 @ 2545 fps
-- 1.49% Leverevolution @ 2625 fps
-- 2.34% WW 760 @ 2660 fps
-- 2.65% WC 845 @ 2618 fps

Mean Radius for all powders tested with the 57 gr. GC, from lowest to highest:
-- 0.56" Reloader 10 @ 2712 fps
-- 0.63" WC 845 @ 2618 fps
-- 0.64" Reloader 10 @ 2545 fps
-- 0.64" Leverevolution @ 2625 fps
-- 0.69" WC 844 @ 2695 fps
-- 0.69" WW 760 @ 2660 fps

Sometimes there was still a little grey in the corner of the grooves. Not the entire barrel, just in a few spots.

A bit of light grey streaks here. Unfortunately I forgot to write down where this was or which load. :lol: This fouling is probably not bad enough to hurt anything but I would would just as soon not see any fouling at all !

Taken near the muzzle at the end of the day, after firing the 46 gr. pills at 2707 fps. Yesterday there were some light grey streaks in this same area so maybe the firelapping is helping a little ?

-- Reloader 10 is best for velocity variation and for accuracy, though the accuracy difference is not significant
-- WC 844 is 2nd best for velocity variation.
-- I have more WC 844 than Reloader 10, so I'm going to use WC 844 for most of my testing in this barrel.

Things To Try Next Time:
-- 4 more firelapping rounds. That'll bring it up to a total of 10 rounds and I'm hoping 10 will be enough.
-- try coated 62 gr. plain base
-- try un-coated 77 gr. GC
-- try different BHN with the 57 gr. GC.

Re: 6mm BR & switchbarrel bolt gun

Posted: Fri Feb 26, 2016 9:15 pm
by mtngun
Last time we settled on WC844 powder (at least for GC bullets), this time we're looking at different bullets, different BHN, and different diameters. Tons of data to report, and some of the data is confusing so you may want to skip the data and scroll down to the conclusions. :lol:

We began the session by firing 4 more firelapping rounds, #280 grit. That makes a total of 10 firelapping rounds through this barrel. I'm inclined to stop at 10.

Here's today's target and I've assigned each group an ID number.

Group #1, 57 gr. GC sized 0.2444": ~25BHN, sized 0.2444" x 0.242", 24.2 gr. WC844, 2.030" COL that seats to 1.992" when chambered. 2673 fps, 0.85% standard deviation, 15 shots in 4.33", mean radius = 1.26". Conclusion: the 0.2444" diameter was a failure. :cry:

A some light leading near the muzzle after group #1:

Group #2, 57 gr. GC sized 0.2435": ~25 BHN, sized 0.2435" x 0.242", 24.2 gr. WC844, 2.020" COL that seats to 1.992" when chambered. 2602 fps, 0.80% standard deviation, 15 shots in 2.91", mean radius = 0.75". This was the same batch of bullets we tested last week and it shot about the same as last week.

Group #3, 57 gr. GC resized to 0.2435": ~25BHN, sized 0.2444" x 0.242", 24.2 gr. WC844, 2.030" COL that seats to 1.992" when chambered. 2736 fps, 0.70% standard deviation, 15 shots in 1.92", mean radius = 0.75". This was the same batch of 0.2444" bullets from group #1 except I resized them to 0.2435". Note that in between Group #2 and #3 I straightened out the chronograph, which was a tiny bit crooked, and that may have caused the velocity to jump up. Anyway the resized 0.2435" bullet shot about the same as the other 0.2435" bullet. Conclusion: this bullet/alloy combo prefers 0.2435".

Group #4, 57 gr. GC with BHN 40: heat treated reclaimed shot, ~40 BHN, sized 0.2444" x 0.242", 24.2 gr. WC844, 2.025" COL that seats to 1.992" when chambered. 2820 fps, 0.76% standard deviation, 15 shots in 2.05", mean radius = 0.51". Best group of the day. 8-)

Group #5, 57 gr. GC with BHN 40 except resized 0.2435": heat treated reclaimed shot, ~40 BHN, sized 0.2435" x 0.242", 24.2 gr. WC844, 2.025" COL that seats to 1.992" when chambered. 2713 fps, 0.86% standard deviation, 15 shots in 2.94", mean radius = 0.78". The wind picked up to 25 mph when this group was shot so that may explain the horizontal dispersion. The vertical dispersion is decent.

Group #6, same load as #4: 2754 fps, 0.86% standard deviation, 12 shots in 2.22", mean radius for #4 & #6 combined = 0.61". Conclusion: the 40 BHN/.2444" combo was repeatable.

Taran report for 40 BHN GC and 0.2444" diameter. Bonus points for doing this at ~2800 fps ! :)

Something dark in the corner of the groove. It's not serious but it's not squeaky clean, either.

Group #7, 57 gr GC with BHN 11: air-cooled WW, ~11 BHN, sized 0.2435" x 0.242", 24.2 gr. WC844, 2.025" COL that seats to 1.992" when chambered. 2590 fps, 1.28% standard deviation, 15 shots in 3.35", mean radius = 0.79".

But some grey color in the corner of the groove after #7. :(

Group #8, same as #7 except boost velocity to ~2700 fps: 25 gr. WC844, 2694 fps, 0.67% standard deviation, 14 shots in 6.1", mean radius = 1.37". Conclusion: BHN 11 GC does ok at 2500 fps but starts shooting wild at 2700 fps.

Definitely leading after #8.

Group #9, 77 gr. GC: heat treated WW, ~25 BHN, 25 gr. WC844, 2.205" COL that seats to 2.192" when chambered. 2608 fps, 0.87% standard deviation, 16 shots in 2.07", mean radius = 0.54". This bullet had been nose sized in the nose die that matches the throat. 2nd best group of the day. 8-)

Group #10, coated 62 gr. bevel base: air cooled WW, ~11 BHN, 2 coats oil based polyu total 0.001" thick, 24.2 WC844, 2.015" COL that seats to 2.003" when chambered. 2532 fps, 2.38% standard deviation, 10 shots in 3.13", mean radius = 0.79". Conclusion: WC844 did not burn well at this velocity.

Group #11, same as #10 but boost velocity to ~2700 fps: 2667 fps, 0.6% standard deviation, 10 shots in 8.89", mean radius = 2.41". Conclusion: WC844 burned fine at this velocity, but the plain base bullet crapped out.

Group #12, 62 gr. flat base coated with water-based polyu: air cooled WW, ~11 BHN, 3 coats water based polyu total 0.00045" thick, 25.0 gr. WC844, 2.015" COL that seats to 2.003" when chambered. 2669 fps, wild accuracy. Conclusion: the plain base bullet crapped out at this velocity.

Conclusions and Observations:
-- the 57 gr. GC @25 BHN definitely prefers 0.2435" diameter
-- but the same bullet prefers 0.2444" diameter @ 40 BHN. :roll:
-- the 40 BHN GC seemed to have an edge over the 25 BHN GC.
-- the 57 gr. GC @11 BHN did OK at 2590 fps but crapped out at 2700 fps.
-- the 77 gr. GC did well :)
-- the coated PB were crapping out at 2700 fps.
-- the coated PB were not compatible with WC844 because you can't drive them fast enough to make WC844 burn well.
-- too many variables to draw conclusions about water-based polyu. The water-based coating turned out thinner than the oil based, and I was comparing a flat base bullet to a bevel base bullet.
-- my chrono velocities may be sensitive to imperfect alignment and it is not bolted down so it tends to get moved around by muzzle blast. Long term I need to figure out a better system to hold the chrono in perfect alignment.

Things to Try Next Time:
-- phase in heat treated reclaimed shot for GC bullets, since the 40 - 45 BHN seems more accurate than 25 BHN, especially at higher velocities.
-- retest the 77 gr. bullet.
-- as time allows make a mold for a 65 gr. GC
-- do over on the oil-based vs. water-based polyu comparison. Try to compare exact same bullet with same coating thickness. Test at 2570 fps and use Reloader 10 instead of WC844.
-- try coating the 40 - 45 BHN GC bullets.

Today was frustrating because many of my experiments did not work as planned. But we are learning what works and what doesn't work and that's the main goal.

Re: 6mm BR & switchbarrel bolt gun

Posted: Sun Feb 28, 2016 10:49 am
by mtngun
40 mph wind outside so it's a good day to stay inside the shop.

The neck-turning attachment for my Forster case trimmer never worked satisfactorily so I upgraded to Forster's hand-held neck turning tool. After making a snug fitting pilot it was quick work to neck turn cases using an electric drill to spin the case. I turned the 0.013" thick Lapau necks down to 0.0125" for a snug fit with 0.2435" - 0.2444" bullets in my 0.270" chamber.


Re: 6mm BR & switchbarrel bolt gun

Posted: Sun Feb 28, 2016 11:47 am
by mtngun
How heavy a 6mm bullet can a 14" twist stabilize? Well, until I have a better theory, I'm going by the Miller Stability Factor and a self-imposed minimum stability factor of 1.25, because the 200 yard jacketed benchrest crowd favors 1.25 - 1.30 and I figure they might know what they are doing. :lol:

My cast spitzers hit that limit around 75 - 80 grains, when cast of wheelweight.

There is a trade-off between twist rate and accuracy at high velocity. The faster an unbalanced bullet spins, the more it wobbles, hurting accuracy. For best accuracy you want to spin your bullet fast enough to stabilize reliably, but no faster. The 200 yard jacketed benchrest boys favor 13.5" - 14" twists for 6mm and 17" - 18" for 30 caliber.

Re: 6mm BR & switchbarrel bolt gun

Posted: Sun Feb 28, 2016 3:21 pm
by mtngun
Concerned about heavy barrel blanks hanging off the not-so-rigid M700 action, I installed a pair of delrin tipped spring plungers on the forend. The theory is that the spring plungers will dampen vibrations. It's just a theory and I have yet to do a comparison test with vs. without the spring plungers. I figure if that if a particular barrel doesn't like the spring plungers, it'll be easy enough to back them off a few turns so they're not touching the barrel.

FWIW the plungers are Jergens #30703, 10-32, delrin tip, 0.5 - 4.0 pounds force.

UPDATE: I no longer use the spring plungers or any other type of pressure at the front of the stock. While there is no doubt that pressure at the front of the stock can sometimes dampen barrel vibrations, the optimal dampening varies from barrel to barrel and from load to load. With this being a switch barrel rifle used to test many different loads, it's not practical to continually tune the front end pressure for whatever barrel/load combo I'm shooting at the moment.