.172 cal Varmint Knocker Slugs

[rsterne]

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....



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)….



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)….  …. 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....



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.... 

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.... 

Bob

[D-RIG]

I like the chart you did showing the different sized slugs
now I would of thought that the 32.7 grain was going to
do good according to the power that your airgun makes ,

I'm looking forward to more of your testing and the results
that you get , good job there Bob .

[rsterne]

Hey, Dale.... I'm super impressed with the quality of your slugs.... and the packing job was SUPERB.... 

I think the 32.7 gr. is just a bit long for the 9" twist, but we won't know for sure until I get a chance to test them at 50-100 yards.... I'm hoping the 30 gr. will work OK, I can push them at nearly 950 fps at the current SSG gap, and I can crank it up from there as well.... 

I think if they were a 6-S Ogive they would chamber a lot easier, because not only would the bearing area be shorter, but the ogive just in front of it would ease the slug into the leade easier.... It would drop the weight about 2.5 gr. though, for the same LOA....

If the 30 gr. work well and the 32.7 tumble, would it make any sense to try something in between?.... I'm looking forward to finding out what the BC of all of these are....   

Bob

[YEMX]

OH man, I need to get my .172 build done!

[Blutroop]

Bob, what’s the oal of those slugs? Now you got me thinking of a 1-7 twist. If your graph is any indication, I should pick up around 25fps by sizing. Now I just need to figure out a retractable bolt.
Dale, I still haven’t used up all those cast you gave me but I got them over 900 fps before breaking my bolt. Are you still taking orders?

[rsterne]

Regarding sizing, the chamber length is probably more important than sizing smaller than 0.172".... In fact, my opinion is that if you have to size smaller than 0.171" to get a 25 fps velocity increase the chamber is likely too short.... You should be able to feel the extra resistance on chambering if that is the case....

The lengths are:

24 gr. 0.470"
24.7 NOE 0.493"
27 gr. 0.516" (3.0 calibers long)
30 gr. 0.559"
32.7 gr. 0.602" (3.5 calibers long)

Swaged slugs from Dale with the same 2-S nose that are longer would weigh....
3.75 cal long (0.645") ~ 35.6 gr.
4.0 cal long (0.688") ~ 38.4 gr.... I doubt if anything much longer would stabilize in a 7" twist....

Corbin make dies with longer ogives for a bit less drag, but they are expensive.... Using a 6-S ogive would reduce the weight of any of the above lengths by ~ 2.5 gr.... Corbin currently have a "ULD" (Ultra Low Drag) set of dies on sale in .172 cal, but I do not know the range of weights they can produce.... They use a Secant ogive instead of a Tangent ogive, so for the same LOA would be a bit lighter than using a 6-S nose....

http://www.swagedies.com/mm5/merchant.mvc?Screen=PROD&Store_Code=CBST&Product_Code=PF_1-HC_172_ULD&Category_Code=Specials

I am pretty sure that die is for FN slugs with a 0.083" flat.... It fits the Hydraulic "H" series press only.... I do not know what else is required to swage .172 cal slugs....

Based on the results with my PCP, I would suggest that if you are considering a twist rate faster than 9", so that you can shoot slugs 0.60" long or longer, that you plan on regulated over 3000 psi, or unregulated at 3600 psi (250 bar) or more, and a barrel of at least 28" (or some equivalent combination).... The heavier your slugs, the higher your combination of pressure times barrel length needs to be.... Otherwise you will not have enough power to get the heavier slugs into the 900's.... My combination of 2800 psi, a 28" barrel, and full bore-size porting (with retractable bolt) is able to get 60 FPE with decent efficiency....

Bob

[Motorhead]

Bob,
I have a TJ .172 in 1-22 twist.
What slugs would be correct for such a barrel ?

[rsterne]

None that I am aware of, much too slow a twist rate.... I have never heard of that barrel.... Are you sure it's not a .177 cal pellet barrel?.... If there is any chance of finding a slug for it, Nick would be the guy to ask.... It would have to be short and light.... Something like 0.22" LOA and 10 gr. might work?.... Dale could make you a few different lengths to try, it should be possible with his dies....

Bob

[Blutroop]

yes My chamber needs work. I eventually snapped my bolt tip as it was so hard to chamber.
Is there a place to buy a reamer that will extend my chamber or does everyone cut their own?
I can’t wait to get my .172 up and shooting again.

[rsterne]

Extending a chamber is a bit tricky.... You may be able to use a tapered pin reamer with the back section ground down at the chamber diameter you want (eg. 0.173" or so)….

Bob

[mackeral5]

Extending a chamber is a bit tricky.... You may be able to use a tapered pin reamer with the back section ground down at the chamber diameter you want (eg. 0.173" or so)….

Bob

This method works very well for a DIY job if you are careful....if you are capable of drilling barrel ports, porting valves, etc you should be able to both modify the reamer and extend the chamber. 

To make the tool, I determine where to size the reamer then chuck in a drill, and run it against my 1" vertical belt sander to turn down the taper behind where I want the cutter to stop.

This is a #7 taper pin reamer modified to cut a .357 lead.  I actually use this to cut the lead and chamber in a single operation....so far I've done this in .257, .300, and .357.

[KnifeMaker]

Scott's barrel according to the TJ's listing is indeed a pellet barrel. He is looking at the bore dia. Rather than the groove dia. Which is what has to be looked at and sized for. + or-


It is .172 bore, .177 grove.

[Hobbyman2007]

The tj’s Barrel I have measure on The big side at .1732” . Drigs slugs are a bit undersized for my barrel unfortunately. I’ll have to cast some 24 grain NoE to test .

[rsterne]

Did you try them, or are you just assuming that they are too small?.... I often find that slugs a thou under the groove diameter work perfectly....

Bob

[moorepower]

Make sure you measure the barrel and twist. I ordered a .25 acp barrel and got a .257 barrel.