GTA
All Springer/NP/PCP Air Gun Discussion General => Machine Shop Talk & AG Parts Machining => Topic started by: CableStop on March 09, 2021, 08:23:02 PM
-
I'm messing around with an idea I've been thinking about for a while, a long range small caliber air rifle capable of shooting slugs at super sonic speeds. I don't know if the slugs would be stable at that speed so it might need to be de-tuned for accuracy but the idea is just to have a rifle capable of reaching those velocities to find out. NSA says their .178 slugs work well in Lothar Walther barrels so I decided to get a LW Marauder to build off of. It got a host of upgrades thrown at it, new air tube, hammer, TSS, HUMA regulator/gauge block, and PEEK poppet. I also ported the valve throat to .25" and TP to .187" then the barrel to .156" since the stock .135" porting restricted the slugs to 550 FPS.
Initial testing has been good, max velocity was 975 FPS with 15 grain slugs which is 31.6 FPE. While I'm happy with those numbers the bug has kind of bitten me and I'm thinking of ways to improve it. I think my next step is going to be to swap the regulator for the one out of my .30 conversion, that will give me a much larger plenum volume, more than double. While I don't think I can use all that extra air, I'm hoping that the larger volume will keep pressure drop low and efficiency high through the shot cycle. The only other things I can think of are to enlarge the barrel port and reduce the diameter of the probe.
Going any larger on the hole will mean it needs to be oval, the next size drill bit would be 11/64 or .172" which I worry will be too large for a .178" slug to pass over. Port size as it stands is 82% of the bore area, I think it could be increased by at least 10% with an oval hole. Before I mess with barrel porting though I have to address the probe, it measures .0915" in diameter which is just about 27% of the bore so it's already a restriction to the flow. I will probably end up making a new bolt with a retractable probe, I don't know how much more material is feasible to remove from the existing one given how small it is.
Any suggestions would be helpful, my dream is to shoot 15 grain slugs at 1300 FPS just to see how they perform. That would be 56 ft-lb which would be insane, imagine a .177 making more power than a stock .25 Marauder. I probably won't achieve that with this gun due to shot count limitations but I think it's physically possible. I know there are formulas based on barrel length and average pressure that can estimate max velocity but I haven't done those yet, once I have a decent idea of the plenum size and pressure I'll do some calculations.
-
I have my JSAR raptor set up in .177 and have shot the NSA 12.5's supersonic being in the 1180 range And some pre production 10.5 NSA @ 1260 fps ....NOT ACCURATE !!!
In an air gun operating on the pressure we do you simply CAN NOT exceed the speed of sound enough at launch to keep said projectile supersonic all the way out to target at realistic distances worth shooting a slug to begin with 75 +++++ yards.
This has the projectile going supersonic then again becoming subsonic during its flight.
You need to read up on what happens when a bullet makes the transition into and out of supersonic speeds.
It simply is not worth it for us air gun shooters ... if it was you would have folks being successful in this venture .. it's not & we dont :-[
ALSO .... own a very very very trick .177 M-rod and got it to go supersonic with pellets in the 8-9 grain range ( using a balanced valve and highly modified overall ) Accuracy is the one thing it DID NOT have at this elevated power.
Good luck and have fun 8)
-
On the note of power .... the .177 JSAR is running right now @ 38 fpe and this with 15-16 grain pellets or slugs. More weight does not make any more power as currently set. Shoots JSB Beast 16.2 or NSA 15's low 1000 fps ( 1040 -1060 fps ) and is deadly accurate at 100 yards or more in low wind.
Big numbers sound good ... just not real practical or even easy to achieve.
-
Are you using the magizine or a shot tray?
Will the jsb 16.2 beast fit a .177 mrod mag?
As for porting, if you know your only gonna shoot slugs then you can go full bore size without interfearing with loading.
-
While it might be possible to come up with a combination of velocity, twist rate and slug that would allow accuracy.... I'd not bet on it happening if you can't keep the slug Supersonic all the way to the target.... In addition, other than the flat trajectory, there are NO advantages, IMO.... The wind drift increases as you exceed about 950 fps, and is the worst at around 1500, not dropping back to what you had at 950ish until you get up to about 2500 fps....
(https://hosting.photobucket.com/albums/oo221/rsterne/Ballistics/.highres/Wind Drift G1_zpsix0tzr8y.jpg?width=1920&height=1080&fit=bounds) (https://app.photobucket.com/u/rsterne/a/ad32680a-7969-443d-ab0e-5a65c7b8979e/p/bac0babb-d1a1-465f-b320-b9aa179eb5f7)
That applies regardless of projectile shape, it is caused by the huge increase in drag in the Transonic region (Mach 0.8-1.2).... why it is called the "Sound Barrier".... In addition, the internal efficiency of a PCP drops drastically as you push past about 1000 fps, unless you use Helium.... That is mostly because the mass of the air pushing the slug becomes greater than the slug itself.... It is simply not worth using all that extra air to go Supersonic and then have the slug fall back below Mach 1 in the first few yards after it leaves the muzzle....
Stick to velocities under about 1050 fps with slugs, and under about 950 with pellets.... and you will be much happier with your results.... If you really want a long range .17 cal, go with a .172 cal shooting the 26 gr. NOE slug (or similar).... Mine does 1050 fps with that (64 FPE) on 2800 psi regulated with a 28" barrel....
Bob
-
Bob - I have read a lot of information you've posted on airgun ballistics, you really know how to present data well. There was a thread where you compiled some info that Nick Nielsen had sent to you which was very helpful in choosing the slug I'm current shooting. I believe you went into more detail about wind drift at 950 vs 1500 vs 2500 in your posts over there. If I ever build a bottle gun I will consider a .17 barrel, but I wouldn't get many shots out of a stock size marauder tube using one right now. Currently maxing the 15 grains out at 996 FPS... so close to 1000 but just not in the cards for today. That's with a bored out gauge block and 30cc plenum regulated at 2500 PSI, it still get's right about 10 shots which is surprising.
JSB Beast pellets do fit in the magazine with some slight encouragement, lengthwise they are fine but the skirt is just a hair too big and they need to be pushed in with something like a small screw driver. I shot them from this gun a while ago and they were faster than the NSA slugs despite being heavier but I haven't shot them recently so I have no good comparison right now. I'll do some tests with them to see, I mostly didn't shoot them because I knew they'd be going too fast to be accurate given the speed the slugs were already moving at. The fact I can go to full bore porting with the slugs is good to know, however I do want this to eventually be turned into something reasonable for recreational shooting after I feel satisfied with the data I've gathered with it as a test bed.
I think I will address the probe first before doing any more to the barrel port, I have a pretty trick retractable probe design I came up with at work today. If it works it should extend and retract the probe in one cycle of the bolt with only some minor modifications to the breech, the bolt design however is probably not one I'd like to rely on in the field, at least without a lot of testing before hand.
I'm aware of the general principles at play as a round falls below the speed of sound, but not anywhere close to knowledgeable on the subject. I didn't know there was a decrease in efficiency as the mass of the air increased compared to the projectile, that's very interesting. Mostly at the end of this I want a rifle with enough adjustment on to be confident I'm getting the best accuracy, if I can use the same barrel to test rounds from 800 - 1100 FPS then I'll know what it likes.
Hopefully this is informative to someone who reads it, I know compared to what a lot of people on this forum are dong my project is pretty small beans. I just couldn't find much information on what kind of power is possible from .177 so I was hoping to put some info out there in this thread.
-
Just an update on how things are progressing. I decided to grind the probe flat so that it's oriented with the thin side down towards the transfer port when the bolt is locked. Measuring it shows a 33% reduction in the area of the probe face, and a 43% reduction in the cross section facing the transfer port. I haven't had any issues with the thin edge digging into the back of the slugs, I've loaded a few rounds from the magazine and pushed them out to inspect the base and none have been damaged.
In the stock bolt, the probe portion doesn't extend far enough back to clear the back edge of the transfer port in the barrel. I need to extend the probe section a little further back on the lathe in order to have the entire TP uncovered. I'm not putting this one on the lathe until I get some spares though.
Using the same 15 grain slugs with all the other settings the same, the velocity jumped to around 1010 FPS, a gain of 15 FPS. I think at this point I need to open up the TP to .177" to see any more real improvement, which I'm planning on doing. I want to be able to lower the regulator pressure a little to increase shot count and opening up the TP should help with that, I'm not really planning on pushing the speeds any further (other than for bragging rights of course).
Speaking of shot count, the hammer setting I decided to stick with got 18 shots averaging right about 1000 FPS, the 19th shot dropped to 988, the 20th to 983. I'm pretty happy with where it is right now, given the set pressure of the regulator is at 170 BAR (2465 PSI). Hopefully opening up the TP lets me drop that back to 160 BAR (2320 PSI) and maintain the same kinds of velocity. Currently on hold until I get more slugs though, I kept forgetting to order them and now I'm pretty low on the 15's so off to buy a few more 400 packs I go.
Finally, a word on JSB Beast pellets, originally they gave a fairly substantial improvement over the slugs but now the margins have narrowed drastically. The pellets still make more power than the slugs but just barely at 36 vs 34 FPE, and they probably aren't accurate at these speeds. I assume at lower velocities the pellet skirt seals better into the bore, but now there is enough energy to expand the base of the slug so it seals the bore almost as well.
-
While you may get away with running a bore-size barrel port on relatively long slugs.... I would watch for loading problems with pellets, as they may drop into the port and end up loading crooked, or getting the bottom carved off on chambering.... If a slug is long enough that the parallel section bridges the barrel port you may be OK, although I have not tried it.... When I need a bore-area port I always use an oblong one, never going wider than 80% of the caliber....
Bob
-
Out of curiosity (and the fact I screwed up my .177 bolt despite saying I wouldn't) I put my 30 cal barrel and breech on the .177 tube, with no other changes the gun shot 50gr JSB pellets at 760 FPS, which is 64 FPE. Just by increasing hammer strike I was able to get it to 800 FPS or 71 FPE but then I ran out of adjustment room, I figure the 20g hammer that's in it is just a bit too light to take full advantage of the 30 caliber. Sort of puts it into context how much power you gain just be increasing calibers, obviously this is a bit of an extreme example but it serves to take most of the restrictions out of the system and focus just on the barrel caliber.
On another note the new bolt is almost done, trying it in the gun revealed that the taper between the bolt and probe is critical. Since the probe is longer than the magazine, there's a point where the projectile is in the barrel but the bolt itself is not in the magazine so the magazine can rotate slightly and the bolt needs to be able to push it back out of the way. In my bolt there is a 90 degree step due to the way I constructed it since I made the bolt itself out of brass and inserted a heat treated steel probe I ground down separately. I'm going to build up the transition with some epoxy and then re-profile it into a taper since the ground probe extends back into bolt a little before it returns to full diameter, leaving a hollow gap at the end which goes into the magazine. The epoxy will fill that gap and extend the bolt section out over the probe a bit further so I have more material to work with.
Here are some pictures showing the 90 degree lip where the probe meets it the bolt and the hollow space around it in the end of the bolt.