6mm BR & switchbarrel bolt gun

[mtngun]

A trial run at sorting bullets by weight. I weighed these after applying the gas check and coating, which is probably not the right way to do it, but it's just a trial run so I'm not picking nits. Mainly what we are looking for is lighter-than-normal bullets that may have internal voids or casting defects that could cause fliers.

These were cast in a 3-cavity mold and one cavity is 0.7 grains heavier than the other two, not unusual for a 3-cavity mold. I doubt if the 0.7 grain difference between cavities will make any difference at 100 yards, but a defective bullet from the heavy cavity will weigh about the same as a good bullet from the other two cavities so weight-sorting won't catch it.

Mainly what we are looking for is lighter-than-normal bullets, because those are the ones that may have internal voids or casting defects that could cause fliers.

The 3 bullets on the right are suspiciously light even though they look good on the outside, so they will be set aside. Perhaps I'll gradually collect "light" bullets until I have enough to shoot some groups with, to see if they are less accurate than normal weight bullets?

Note that the 59.4 gr. bullet had a poorly filled out band. It will be added to the "light" bullet collection, too.

[mtngun]

OK, today we tested the revised 60 gr. GC bullet. There's been so many revisions that it's hard to keep them straight, so let's just call them #1, #2, and #3. #1 and #2 are secant ogives, while #3 is closer to a tangential ogive. The idea behind #3 was to give the ogive a little more support.


Going into this I had settled on the following load factors.
-- 60 gr. weight
-- 40 BHN alloy
-- OBPU coating (for the time being, more coating tests to come)
-- 25.1 gr. WC844 (not necessarily the most accurate powder but I had a couple of jugs that were looking for something to do )
-- full monty Ardito-style tapered glove fit, seated with only the gas check in the case
-- CCI #41 primer (not necessarily the most accurate primer but I have a bunch of them )
-- free floated barrel

Today I started on the bottom row of targets, and went left to right. Then the middle row, and finally the top row.

As you can see the new improved #3 bullet didn't do all that great. It's 2.02" average was about the same as previous tests with the #2 bullet. That didn't seem right because several weeks ago this barrel seemed able to shoot 1.5" without trying hard. I wondered what had changed? Well, I am always changing things but most of those changes are tested very carefully.

One big change was the bedding. It used to have the spring plungers on the forearm but then I switched to a free floating barrel. The free floated barrel did not seem to hurt accuracy but then again the spring plungers had been tweaked numerous times so there was no cut and dry comparison.

I wanted to rule out barrel vibes causing the fliers, yet it had to be something simple enough to not require time-consuming tweaking each time the barrel is swapped. So I cut a 3/8" wide strip from an old, cheap-style mouse pad. Not the hard plastic kind but a soft foam pad. The strip was pinched between the barrel and the tip of the stock. Because the mouse pad material is so thin and so soft, it exerts very little pressure on the barrel. The hope was that the soft foam rubber would provide just enough contact to dampen vibrations. Perhaps it was just coincidence, but groups did shrink a bit with the mouse pad bedding.

I ran out of the #3 bullets so I shot one last group (top left) with the #2 bullets, which had shot so poorly last time. Darned if that wasn't the best group of the day!

Then for a sanity check I finished with some of the 77 grainers that I have tested on previous occasions, which usually run 1 3/4" or so. This time I cut the powder from 26.5 gr. to 26 gr. hoping that would help accuracy, but it did not, averaging 1.98" and with more velocity variation than last week's 26.5 gr. load.


A close-up of the final 60 grain group, using the #2 design bullets for the first 9 shots and a #3 bullet for the 10th shot. Yes, it hit the magic 1 MOA though just barely, and more than a little luck was involved. Ironically, the mirage was at its worst for this group -- I was basically shooting at a dancing orange blur.

It may have been luck but I'll take it!


Trivial stats (I include them because I use these posts as a form of record keeping):
-- average velocity with 25.1 gr 844 / 60 gr. GC = 2772 fps
-- average velocity standard deviation with 60 gr. GC = 0.82%
-- average 10 shot group with free floated barrel, 60 gr. GC = 2.02" for 5 groups
-- average 10 shot group with mouse pad barrel, 60 gr. GC = 1.46" for 4 groups

Conclusions:
-- the #3 design, with it's tangential ogive and slightly longer bearing length, did not seem to be more accurate than #2 or #1.
-- the mouse pad bedding may have been an improvement but I don't think I "proved" that. At a minimum the mouse pad bedding did not hurt accuracy so for now I'll continue using it.
-- overall I was disappointed because I haven't made any accuracy progress for several weeks -- I seem stuck at 1.5 MOA.

Things To Try Next Time and Down The Road:
-- I'm itching to move on to other things so for now the mouse pad & 60 gr. GC will be the control load.
-- test the control load with different band & groove layouts.
-- test the control load with different coatings.
-- revisit coated plain base.

[mtngun]

I used the 6BR to do another coating shootout, posted at the CBA forum.

Update: I have been told that the flaking and brittleness I observed on my powder coated bullets could be a sign that the coating was not cured long enough. I followed the manufacturer's curing instructions but .... every part is different, and time required for the bullet to come up to curing temperature has to be allowed for.

So anyway, I will experiment with a longer curing time and see if that eliminates the flaking.

[mtngun]

I admitted defeat with the Hi-Temp silicone powder coat but polyester powder coat perked up after adding 20 minutes to the recommended cure time.

[mtngun]

I cut an inch off the stock. Reason being, this will be shot free recoil 90% of the time (with any barrel that kicks no more than a 30 BR) and you have to keep your shoulder one or two inches away from the butt of the stock when shooting free recoil. The stock is still long enough that it can be gripped if required for harder recoiling cartridges.


I also added this hi-tech return-to-battery system. You can laugh but it protects my shoulder when shooting 7BR & 30BR, plus it seems to help the rifle track a little better -- the rear bag doesn't get knocked around as much as it used to.


UPDATE: I no longer use the rubber bands. but out of consideration for my shoulder I did install a recoil pad.

[mtngun]

I've never been satisfied with the way the rifle "rides the bags," because it bounces around too much and the crosshairs do not return to the bullseye when the rifle is returned to battery. One problem has been that the Protektor cordura front bag does not hold its shape. It's constantly "shape shifting" and seems to somehow move the sand from one side of the bag to the other side.

So I upgraded to Protektor's top-of-the-line leather bag, with the hope that leather will hold its shape better than cordura? We'll see. Considering what I paid for the leather bag, I'll be disappointed if it doesn't cut my groups in half.


In case you are wondering why the windage cable is clamped to the bench, that serves to keep the cable from flopping around when you turn the windage knob. Without the clamp, it took nearly a full turn of the knob to take up the slop in the cable. Now it only takes perhaps 1/8th turn.

[mtngun]

The fancy new leather front bag does work better than the old cordura bag. It holds its shape and so far has not needed any attention, unlike the cordura bag which had to be re-shaped several times a day.
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Here is one of today's better targets. For deets check out my coating thread on the CBA forum.

[mtngun]

Since I use these threads as a way to keep notes on load deets, I'm going to post this photo showing the diameters of a nose-sized 75 gr. bevel base that shot decently in a previous experiment. When I am setting up the nose die I refer to these dimensions and try to adjust the sizing depth to duplicate them.

FYI "FB" refers to the front band. "B" is the bottom band. "2" is the second band from the bottom, "3" is the 3rd band from the bottom, "4" is the 4th band from the bottom, and "FB" is the front or top band.

[mtngun]

Powder porn. The VV is N133. I couldn't work up enthusiasm for buying a jug of 4198, but there was a dusty 80's vintage can on the back of my shelf. It may take me months to get around to testing these powders.

[mtngun]

A lucky target with the 75 gr. bevel base. If only I could do this every single time I would be pretty happy, but alas repeatability is elusive.


Also, I modified my nose-sizing lubrisizer. Now I can use calipers to dial in the sizing depth. Repeatability seems to be about 0.003".

Before modding:


After modding: the depth stop locknut had a habit of working loose and allowing the depth to drift in mid-batch so I drilled and tapped for a locking screw. You can't see it but there is a little piece of copper between the locking screw and the depth screw to protect the threads.