.510 cal bullpup development project

[lloyd-ss]

This thread will probably run for a while as I muddle my way thru the development and build of a .510 cal bullpup repeater. This is the first gun that I have built for myself in years. Way too long overdue.     A good hunting gun, even though I am not much of a hunter.  Light weight (I hope), compact, powerful, with several good, consistent shots.  Something that will deliver at least 350 foot pounds at the target.  I just can’t get too excited about a .45 or .50 cal that can only muster 200 fpe at the muzzle or that gets “2 or 3 good shots with a follow-up.    ”  Or a hunting gun that is so long or heavy that you can’t lug it around or use it in the woods. Maybe I’ll get where I’m headed, maybe I won’t,      but it will be fun.   

I’ve been working on it for a while now, and I don’t have any nice 3D models.  I wish I did, but I don’t.  A lot of 2D autocad stuff, though. I am zeroing in on the power plant right now, and have a concept development gun..... a developmental model,  going.  Strictly strap down to the bench, but the power and efficiency are coming along nicely.  Then you guys are doing all this work with Peek and Titanium and steel and  valves and balanced valves.  So, I’ve been sucked in and have to explore some of that, too. Thanks for enabling the affliction.

Here are the current specs on the development model, even though the specs are very fluid.

.510 cal barrel from Mike Sayers, currently 43” long, but that will be cut back to a more suitable bullpup length after I get the power plant figured out.
Shot reservoir 475 cc
Valve throat .568 dia
Valve margin O.D. .610
Valve stem .187 dia

Cross sectional areas
Caliber .510 = .204 sqin = 100%
Throat less the stem = .226 sqin = 111% of bore
Exhaust port = .197 sqin = 97% of bore
Transfer plate = .219 sqin = 107% of bore
Total volume of transfer port is 5 ccs

Hammer weight 130 to 190 grams
Stroke 2 to 3 inches
Spring – takes 2 hands to cock it

Current performance
The gun’s velocity is now maxed out at about 2650 psi.  I think the limiting factors are the hammer and valve, which seems to be the topic du jour here amongst the geeks.  And that is perfect because if we can’t figure it out, who will?
Typical shots from 2650 psi
247 gn, 1072 fps,  630 fpe,  efficiency .89 fpe/cuin
318 gn,  984 fps,   684 fpe,  efficiency  .96 fpe/cuin
414 gn,  883 fps,   717 fpe,  efficiency 1.01 fpe/cuin
446 gn,  857 fps,   728 fpe,  efficiency 1.20 fpe/cuin

Of course, the velocities and efficiencies can all be pushed around by a multitude of tricks, but this is a starting point.

Here are some pictures of the valve.  The return spring is on the hammer end of the valve stem, not in the air tank.  The tank is short and fat to dump quickly into the valve. The air flows from the tank to the bullet in a “Z” path, rather than the usual U-turn.







Finally, here is a chart of the probable internal ballistics of the 318 gn shot. The chart represents the actual recorded data of that shot. You will notice that valve does not close until the bullet is almost 21 inches down the barrel.  If all parameters are kept the same, and the barrel is shortened by 10” to give a 33” length, you can see that the velocity will drop to 920fps and the energy will drop to 597fpe, but exactly the same amount of air will be used.  Therefore, the efficiency will drop from .96 fpe/stdcuin, to .84 fpe/stdcuin. So that will certainly be one of many challenges.



Thanks for bearing with me in this long winded intro.
Lloyd

[ezman604]

Most excellent news Lloyd. Very glad to see you in the shop doing a project for yourself. As I told you, if you need or want a BETA tester, I'm your man.

I am placing game cameras this year and rebuilding some stands. I will most likely start deer hunting again. That would be a sweet gun to take my first buck with.
Keep us informed on your progress and good luck. And enjoy making metal chips!!!
SAFE & Happy Shooting!!!!
Dave
 

[rsterne]

Excellent project, Lloyd.... I'll be keeping a close eye on this project, I'm sure we will all learn from it....

Bob

[lloyd-ss]

Dave,
I will keep you in mind when that time comes!

Bob, The first thing I want to do is replace the valve stem and head.  The easiest way would be to make another 2-piece valve with the 3/16 stem threaded into a steel head.  I'd finish machine the head after it was Loctited to the stem.  It probably wouldn't be durable enough for a permanent solution like the little engine intake valves you found, but it ought to show an improvement if one is to be had.

The current valve head is fairly robust, with the stem threaded deeply into the protuberance in the head. I am not expecting much of an improvement, but maybe I'll be surprised. I feel like I'm just delaying the inevitable conversion to a pressure balanced valve.

Lloyd

[lloyd-ss]

I made a new all-steel valve to replace the glass filled Delrin one.
Hopefully I'll be able to reduce the hammer strike, but I am skeptical.  I hope to be proven wrong

Have not even aired it up yet, so I don't know how badly it might leak the first time.
It weighs 15 grams vs 20 grams for the bulky Delrin one.  Three piece construction all screwed firmly and Loctited together.
The head on this one is only .626 dia closing onto the .568 throat.

This is a pre-hardened 4140 steel head seating on 6061T6 aluminum.  I know Sean saw some galling with a similar arrangement.  This is just a developmental model, so hopefully I can learn something before it starts having problems.

iThe load on the valve head at 3000 psi is about 850 pounds     (YOW) and the loading on the margin is in the neighborhood of 23,000 psi.  Hmmmm.  That's significant.

Here are some pics comparing the old to the new.  Testing will follow. Any bets    ?
Lloyd

[Big Bore Bart]

  You need to polish the seating surface of the stem to at least 1500 grit.   Then you stand a better  chance of first try sealing. 

[rsterne]

Just a quick thought.... when lapping valve in automotive engines I never used anything remotely that fine (400 or 600 grit?), and the final "lap" was just a clean seat with a film of oil, reverse-lapped by hand with a dowel and suction cup, just to "prove" the seal, which appears as a dark line on both surfaces.... Cranking the engine over to test the compression only results in 100-160 psi (depending on valve timing), which apparently, in conjunction with the valve spring, seals the valve well enough that it doesn't have blowby and end up burning during combustion, which is the result of any significant leak....

It's possible I suppose that such valves "always" leak, but the % loss is so small as to not cause problems, whereas any leak for us shows up as a constant air loss.... but it makes me wonder why achieving a seal should be so problematic for us?....

Bob

[QVTom]

the load on the valve head at 3000 psi is about 850 pounds     (YOW) and the loading on the margin is in the neighborhood of 23,000 psi.  Homonym.  That's significant.

Lloyd, you are just past half of the yield value of the seat which seems adequate at fist glance.  If you know the rate of closure along with the valve weight and travel, a peak loading could be determined. The static vs dynamic loading would helpful in determining the suitability of the valve materials.

Tom

[rsterne]

Tom, the static loading Lloyd quoted is the value for the highest force involved, which occurs after the valve has sealed and the pressure on the exhaust side bleeds down to zero.... It is the force the hammer has to overcome to "crack" the valve from the seat.... The force then quickly drops to a value of about 90 lbs., which is the closing force due to pressure on the 3/16" stem plus the spring when the valve is open.... That force remains relatively constant until the valve approaches the seat on closing, at which time it increases briefly, and then once a seal is made it returns to 850 lbs. while compressing the seat material....

The closing rate would be determined not by the mass of the valve poppet, but the poppet plus hammer, being accelerated towards the seat by that 90 lbs., and then at the very last instant an exponential increase in force to the 850 lb. value.... Since the valve lift as a function of time is a parabolic path, with height = lift and length = dwell, yes it would be possible to calculate the closing velocity, knowing the weights and forces involved.... As a rough guess, you can probably use a lift of 1/8" and a dwell of 0.002 sec. (so 0.001 sec. for the closing half of the cycle), a combined weight of 180 grams, and a force of 90 lbs. to get a close approximation.... Of course once the valve poppet makes first contact with the seat, the hammer weight is no longer a part of the momentum....

Bob

[lloyd-ss]

Let me ponder all this.
Attempt # 2 and the valve "almost" seals.  Much better than the first fill.
More work to do.

[QVTom]

Bob,

I have made some bad assumptions.  I've gained most of my knowledge about PCP valves from reading your's and Lloyd's posts over the years, not much on actual doing.  I hope to get to the doing one day.

If I understand correctly, during the valve closing, the hammer is sill in contact with the valve stem, basically being driven back against the spring  The closing force, due to the area behind the valve, is much less until the back pressure starts to drop at the near closed point.

If I understand your point correctly, the difference between the dynamic and static force on the valve seat is very small and battering is unlikely.  Can we discount it all together?

Thanks Bob, I learn from every post.

Tom

[rsterne]

The hammer should remain in contact with the valve stem throughout the entire round trip from seat to seat, as there is always a force decelerating, and then reaccelerating it.... Unfortunately, after the poppet seats, the hammer is free to recompress the hammer spring and cause air-wasting hammer bounce on the return trip if it reopens the valve after the Elvis (the pellet) has left the building (the muzzle).... The major closing force is the valve (reservoir) pressure acting on the area of the valve stem, with minor contributions from the spring and parasitic drag on the head (which can be thought of as the pressure diferential between the reservoir and the exhaust port or barrel)....

I have never calculated the dynamic forces due to the closing momentum of the poppet, but would be surprised if it produces a value larger than the static opening force....

Bob

[bradyman1]

Very interesting work Lloyd. Good luck. I would love to see one of you boys build a usable pcp shotgun one of these days.

[Big Bore Bart]

    Bob the automotive stuff is a little coarse,  the criteria for 1275psi + steam and water valves is a lot higher.     I have lapped boiler safety valves with 2500 grit diamond dust.  They have to stay bubble tight to 2000 psi under test.

[PakProtector]

A 12-syring set of diamond lapping compound is about $25 through Chinese dealers on ebay. Very remarkable stuff, and down to 1 micron grit IIRC.
cheers,
Douglas

[rsterne]

bradyman1.... Standby, that's the next thing for my Hayabusa, it will soon be a .410 shotgun.... but I won't hijack Lloyd's thread with it, you will have to wait until I get a little further along with making parts, which shouldn't be long....

Bob

[lloyd-ss]

Well, it took some effort to get the new steel valve to seal.  A couple of tries with the fine grit valve grinding abrasive.  I have some Clover lapping compound, but it is 220 grit, which is no finer than the valve compound.  The only other thing I had on hand was some Turtle Wax Rubbing compound.  I don't know what the grit is, but it was too big of a jump in fineness from the valve grinding compound t the rubbing compound.  After about 8 attempts I finally got it to seal.  Then I put a little moly paste on the valve and seat and twisted them to help avoid any galling or metal transfer.

Testing with the new steel valve
Well, I didn't get what I was hoping for, but I got about what I expected.  I have already concluded, maybe prematurely, that the glass filled Delrin head actually performed pretty decently.

For the first two shots, I reduced the hammer weight, and the spring force. The chrony results were slower than with the glass filled Delrin head.  So I put the hammer weight and spring force back to where they were when I maxed out the Delrin valve. So, still hitting the valve harder than I think is prudent.

What the steel valve  DID allow me to do, was open the valve about 200 psi above where I could before. So that is something good.

Here are the results, all shots taken at 2850 psi, which seems about the max for tonight's set-up:

average of (2) 248 gn bullets, 1080 fps, 642 fpe, .70 efficiency

average of (3) 318 gn bullets, 1003 fps, 711 fpe, .85 efficiency

average of (5) 445 gn bullets, 863 fps, 741 fpe, 1.00 efficiency

So even though I was able to shoot the bullets faster, the efficiency suffered. More work to be done.
Lloyd

[grumpy]

741 fpe ?   

[lloyd-ss]

741 fpe ?   

Yup, they are singing along. Douglas, did you put something special in the lead when you cast these bullets?   
Remember though, the barrel still needs to have 8 to 10 inches amputated.    That ought to slow it down some.   

[SeanMP]

Lloyd

Excellent project. I really like your TP flat on the top of the tube. I'm doing something similar on a 357 concept build right now. The only difference is I have extended the flat to encompass the whole breach.

I looked over your numbers from last night and compared them to the numbers you had posted in your previous thread. What I'm seeing is your now slinging 320gr bullets at a similar velocity to what the 200 gr bullets were previously and picked up +- 50% increase in energy.

Is this not significant?

Bob
I'm still not convinced the hammer and valve follow each other after the collision. I have never seen this phenomenon in any other mechanical system. And I spent some time trying to replicate it using opposing springs and ball bearings in a clear tube. In every case I have always observed an elastic collision and the struck object bouncing away from the striking object.