.50 caliber airgun

Justsomedude

A few months ago I bought an Umarex Hammer 50 cal airgun that's touted to push 250 grain lead slugs (they look like minie balls) at 1000fps. It actually lives up to its reputation and does just that. It's a precharged pneumatic with a tank and regulator so you get 3 shots and 4th at 90% velocity.

Well this got me wanting to build my own to see if I can achieve a higher velocity. I started with a 500/510 1:28 twist green mountain barrel blank, some aluminum, a tank and regulator. I also skipped out on having to build a sear/grip assembly by using one from a Crosman 13xx.

I made a side lever cocking system that opens and closes the bullet probe which also seals the back of the chamber and designed my own balance valve that takes no effort to open, even against 3000psi. The tank holds 4500psi and the shots are regulated at 3000psi each. I'm still working out plenum volume to maximize shot count and velocity. I have to find the happy medium.

Forgive my clamp on the plenum, I got anxious and wanted to test fire it and to do so I thought I'd see it's max performance so I took out the regulator and replaced it with another that I took the guts out of, so basically a single shot that would dump the whole volume from the bottle and all.

Hooked up the magnetospeed and fired a 250 grain hard cast.

I over exceeded my expectations!

sakodude

That's pretty cool.

CHIRO1989

Dang, that is cool, got any lighter bullets to try?

Justsomedude

The lightest I have are the 250 grainers. I also have some 388 grainers that I need to try as well. As it is with the 250's, it's just over 1100 foot pounds of energy. 

Big Al1

Great toy to play with during an ammo, primer,  powder shortage!!

Gene L

How about the sound?  That performance is wild!

CHIRO1989

Justsomedude said:

The lightest I have are the 250 grainers. I also have some 388 grainers that I need to try as well. As it is with the 250's, it's just over 1100 foot pounds of energy. 

They do make some lighter bullets:
https://topairgun.com/product/185-grains-air-venturi-50-cal-hollowpoint-50ct/

Japhy

that is a serious device!

by my quick calculations that is 1114 ft/lbs

and supersonic so there was a crack

Japhy

one more thought

with a gas check  bullet and a larger diameter valve the velocity should go significantly higher

Justsomedude

Gene L said:

How about the sound?  That performance is wild!

It's definitely supersonic. It also sounds like a centerfire being shot.

Japhy said:

one more thought

with a gas check  bullet and a larger diameter valve the velocity should go significantly higher

I really can't ask too much more out of this thing. I designed amd made the valve too which was quite a feat because most airgun valves suffer from valve lock with high pressures. I made mine a balance valve design so that it's impossible to lock. It also dumps as much air as I can put through it, volume and pressure wise.

CHIRO1989 said:

Justsomedude said:

The lightest I have are the 250 grainers. I also have some 388 grainers that I need to try as well. As it is with the 250's, it's just over 1100 foot pounds of energy. 

They do make some lighter bullets:
https://topairgun.com/product/185-grains-air-venturi-50-cal-hollowpoint-50ct/

I will be trying those. I'd love to see what velocities I can get with different weight bullets.

Japhy

cone out the back of the bullet and easily take 50gr off a 0.50 and improve the gas seal at the same time....then you dont have to wait for a shipment of bullets!

Freezer

That is just too cool! Lewis and Clark would be impressed.

The wheel has been reinvented.

Justsomedude

I am definitely very very proud of this thing. My goal was to push these 250 grainers at 1150 and I thought I was being optimistic about that. I about needed a change of boxers when I read the chronograph and saw the 1417 number. From what I'm researching this is the most powerful .50 caliber air rifle at the moment. I also want to tinker with lighter sabots in it.

Japhy

dude that is just first rate machine work! kudos!

on getting more shots per charge

the volume of the barrel increases by the square of the diameter radius  i.e a .25 barrel the same length as the .50 barrel needs 1/ 4 the gas to get the same pressure.

yep I know you just bought a .50 barrel

Justsomedude

Thank you. I'm discovering interesting numbers by trial and error. When I fire it unregulated at 2500psi, it is dumping about 31 cubic inches of air in 1 shot which is where I got the 1417fps. I fired it today at 3000psi, regulated only dumping 5 cubic inches and it achieved 1397fps with a few more shots left over. I'm going to have to come up with the proper pressure and plenum volume to achieve the highest velocity and also highest shot count because it's a pain to pump back up. I have a high pressure pump but it takes a few minutes and I have to give it breaks to cool. I'm going to buy a big air tank for testing and tethering at the range. I could technically fire it at 4500psi unregulated just to see it's maximum possible performance but these pressures scare me a bit lol.

Bigslug

Tell us more about the HEAVIER bullets!

Justsomedude

Bigslug said:

Tell us more about the HEAVIER bullets!

They are .510 diameter, 388 grain slugs that I purchased from Nielsen Specialty Ammo. I haven't put them on the chronograph yet but they make the rifle recoil like a 50 caliber muzzle loader. When I first started building this thing I wanted to see how hard it was to push one of these down the bore of my barrel so I did so with a heavy oak dowel and hammer. It took so much force that it worried me that air wouldn't be able to do so. Boy was I wrong lol. 

Nielsen is top notch quality airgun ammo btw. Pic from website.

Justsomedude

A few pics along the way.

Japhy

Did you design / build / modify the trigger mechanism?

A 388gr slug is going to hold a lot of energy for a long distance. Wonder what a similar setup in .30 would do with a high ballistic coefficient slug like 300 BO

This is just too cool

bullsi1911

That is wicked. Well done!

Justsomedude

Japhy said:

Did you design / build / modify the trigger mechanism?

A 388gr slug is going to hold a lot of energy for a long distance. Wonder what a similar setup in .30 would do with a high ballistic coefficient slug like 300 BO

This is just too cool

I used the grip/sear assembly from a Crosman 1322 pellet pistol. It saved me alot of time and effort. I designed everything else off the top of my head. If I write plans down on paper they never work. I have to just daydream, visualize it and then build it, making improvements as I go. I'm not a hunter but I almost want to drop a buck with this thing lol. I also considered making different caliber barrels because the barrel to reciever fitment is quite simple. I have a feeling that it would seriously push a .357 bullet.

bullsi1911 said:

That is wicked. Well done!

Thank you!

GrapeApe

Yeah, a 0.357" would be awesome.
Soft cast, or swaged 158gr SWC HP, OR, to make bigslug happy, something like a 170-180gr LBT type slug

IMPRESSIVE!!!!

Gene L

Japhy said:

one more thought

with a gas check  bullet and a larger diameter valve the velocity should go significantly higher

Don't see the need for a gas check bullet. They only are used to keep flame from vaporizing the rear of a bullet.  Or am I missing something?

Justsomedude

I cant use a bullet with a rear depression or skirt anyways because my probe has to push the bullet just passed the pressure port in the chamber. I guess I could manually push it there if I really wanted to though.

Justsomedude

Japhy said:

Did you design / build / modify the trigger mechanism?

A 388gr slug is going to hold a lot of energy for a long distance. Wonder what a similar setup in .30 would do with a high ballistic coefficient slug like 300 BO

This is just too cool

Hope this makes it a bit more clear. I made the brass hammer, spring, tube and mount that the grip assembly mounts to. It smacks the stem of my balance valve. Shown uncocked and cocked. 

Bigslug

Seems like cast rifle bullets in .358, .41, or .43 caliber.  Kinda liking the last option - you could potentially be playing in the performance envelope of some serious BP-era military rifle cartridges.  An 11mm Mauser or Gras with no gunpowder would be an impressive thing.

Diver43

That is Awesone
Looking forward to seeing the results as you fine tune it

Gene L

How about recoil?

Japhy

Gene L said:

Japhy said:

one more thought

with a gas check  bullet and a larger diameter valve the velocity should go significantly higher

Don't see the need for a gas check bullet. They only are used to keep flame from vaporizing the rear of a bullet.  Or am I missing something?

This will get quite hot for an instant.  diesel engine cylinder pressure is just under 1000psi, this is 3x that pressure. I remember Daisy came out with a gun that used a small amount of propellant attached to the pellet that was ignited by chamber pressure heating.  they canned it when BATF or whatever it was in that era ruled it was a firearm. going to 2000psi in a few milliseconds from atmosphere will absolutely ignite black powder or for that matter smokeless powder...no primer needed!

Justsomedude

Gene L said:

How about recoil?

It actually has significant recoil. It's comparable to a muzzleloader of the same caliber.

Wambli Ska

Now I’m drooling all over my phone… thanks 🤬