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PostPosted: Wed Mar 25, 2020 4:00 pm 
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Joined: Sat Nov 03, 2007 4:19 pm
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Location: Coalmont BC
As you may remember, I have a .172 cal version of the 6mm regulated PCP I built a couple of years ago.... It has a 28" TJ's barrel and runs regulated at 2800 psi, and is capable of over 60 FPE.... Here is a photo of it....

Image

I was recently chatting with Dale at Varmint Knocker Slugs, and expressed my interest in trying some swaged HPBT slugs that were heavier than the cast NOE slugs I have, which in their HP version weigh 24.7 gr.... This is because my gun can launch those at over 1050 fps.... Dale sent me 50 slugs each of 4 different weights, about every 3 grains from 24-33 gr.... Here is what they look like, compared to the cast NOE slug (second from left)….

Image

Dale slugs are a rebated boattail design (RBT) using a 2-S Ogive with 50% Meplat, and with a HP of about 1/16" in diameter, almost identical to the NOE slug.... Overall, they are quite similar in shape, except that the NOE is a gas check base instead of the RBT.... In addition, the nose of the NOE slug is a slightly smaller diameter ahead of the start of the bearing area.... It turned out that was important when I started chambering all the slugs for testing.... The shortest of the swaged slugs and the NOE chambered with no noticeable "shove" on the bolt required to close it.... However, the 27 gr. swaged presented a slight resistance to chambering, and the longer ones were worse.... with the 32.7 gr. slug requiring a pretty hard push on the bolt to seat it.... The chamber in the barrel was cut for the NOE slug, and needs to be longer for the longer Varmint Knockers....

The purpose of todays testing was to get some idea of the velocity each would produce, and to see if there was any noticeable tumbling when shot through my Chrony…. It turned out that the longest swaged slug was so hard to chamber that it didn't even fire at 2800 psi, requiring cocking and firing 3 times before it even discharged (at about 240 fps)…. :shock: …. I sized it to 0.171", and it was easier to chamber, and fired OK, but at a lower velocity than I expected.... When sized to 0.170", it chambered fine, and the velocity was in the ballpark I expected given its weight.... I tested all five slugs, both as swaged or cast, and also sized to 0.171" and 0.170".... Here are those results....

Image

As you can see, when sized to 0.170", the velocity decreased in pretty much a straight line.... The highest FPE (61.2 FPE) was obtained with the 27 gr. slug, but the 32.7 gr. slug was still within 4 FPE.... When sized to 0.171", the longer slugs, which still required some effort to chamber, lost some velocity, with the 32.7 gr. slug down to 50.4 FPE.... As swaged, at 0.172", the 27 gr. slug lost about 50 fps, the 30 gr. about 150 fps, and as I said earlier, the 32.7 gr. would not even fire.... Obviously, the longer slugs were jammed so hard up against the rifling that the rather small force available of 65 lbs. at 2800 psi was not enough to even launch them.... :shock:

This is not a criticism of Dale's slugs in any way, it is a barrel problem.... The chamber is not long enough to allow them to load properly.... This testing did show up one other problem, however, which I did expect.... The impact holes from the longest 32.7 gr. slugs were not straight at the 7 yd. range to my backstop.... I will confirm this when I get a chance to shoot them outside, but it would appear they are too long for the 9" twist of this TJ's barrel.... The holes from the 30 gr. and shorter slugs were round, but whether they are stable at longer ranges remains to be seen....

With the Motel being closed this spring due to COVID-19, I hope to get out and do some shooting.... My new LabRadar is awaiting testing, and with a little luck I will be able to report on both the accuracy and BC of all these slugs before the end of May.... 8)

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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PostPosted: Sat Mar 28, 2020 5:52 pm 
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Joined: Wed Jan 05, 2011 12:35 pm
Posts: 6421
Location: P.G. B.C.
Good, interesting post, Bob.
I would expect the 30gr. and 32.7gr. to be both a bit long for 9" at sub-sonic speeds.
Been wondering about the seemingly critical nature of the sizing. Likely solely due to
the long bearing surfaces of their sides & fit in the chamber, of course. I would think
narrow driving bands like the second bullet might be less critical of actual size.

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Daryl


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PostPosted: Sat Mar 28, 2020 8:23 pm 
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Joined: Sat Nov 03, 2007 4:19 pm
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Location: Coalmont BC
Absolutely correct, and a bullet like my BBT's, with only the back of the nose and one rear drive band, are even easier to chamber.... However, that is one of the drawbacks of swaging, with the big advantage being that the length of the die is easily adjustable, so the weight can be easily changed.... The die that Dale has can go from less than 10 gr. to over 50 gr.... :shock:

If the 30 gr. works, and somebody had a bucket of money ($630 USD) to drop on a set of dies with a longer nose radius, a slug the length of the 30 gr. with a 6-S Ogive would be about 2.5 gr. lighter and have a much shorter bearing surface (similar to the 24 gr. above), so a lot easier to chamber....

Image

It should have lower drag so a better BC than the 27 gr. in the OP above will have.... maybe as good as the 30 gr. above?.... Lighter and with a the same BC would be a good tradeoff for a varmint bullet.... 8)

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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PostPosted: Sun Mar 29, 2020 1:06 pm 
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Joined: Wed Jan 05, 2011 12:35 pm
Posts: 6421
Location: P.G. B.C.
In case others don't know, a bullet with driving bands, all else being equal to a bullet without, will have lower BC due to
atmospheric drag inducing disturbances from the bands. More bands, more drag. It's all a tradeoff. Longer bearing surface
thus increased drag inside the barrel and more critical diameter.
It's quite a juggling act, but that is ballistics in itself.
Bob, you are way ahead of me on this stuff, but what you said about the changes makes sense. Too bad it's at a time when
we're watching the pennies.

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Daryl


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