Making a Bulk-Fill Cap for a Crosman 160 Part--2

The slow saga continues.

Didn't think that there was enough material in the plug made in part 1 for 1/8" NPT female thread to fit my bulk-fill adapter. Rather than start again from scratch (and buy another specialty tap to match the gas tube threading) I thought that a larger diameter head added to the front of the plug seemed to be the easiest way to salvage the project. So, another piece of 0.750" dia. drill rod is cut.

Reduced just less than half the OAL to 0.379" dia.

...and threaded to 3/8"-24.

The new head for the plug. Why 3/8"-24? Well, it looked like the largest thread size I could comfortably cut into the plug and I had both the required tap and thread cutting die--so no additional capital outlay was necessary.

Need to seal the two pieces together so the CO2 won't leak. A small, home ground parting tool made a relief cut at the base of the male threads for a small o-ring. Wrapping several turns of teflon tape around the threads would have probably worked, too, but I'm a glutton for punishment.

The front of the plug was initially left long to provide a holding surface to cut the o-ring grooves. Shortened it up a bit with the parting tool.

Again, lots of cutting fluid and slow turning speeds saves the blade.

Through drilled the plug with a 1/8" bit.

Then, drilled partially through the front third of the plug with a 21/64" bit and counter bored. This left a stepped hole of two sizes. No pictures for some reason? Not shown: Tapped the hole 3/8"-24 followed with a bottoming tap. A bottoming tap typically leaves about three threads only partially formed at the bottom of the hole. So, I ground off the taper on a spare tap and put full-size threads right to the bottom.

Used a tiny boring bar to cut a seat for the o-ring into the front of the plug to seal the threads.

The small o-ring to seal the halves together is a P7. The two larger o-rings that seal the gas tube are 112's.

O-rings on...

and pieces screwed together with the locking ring in the middle.

Installed, again, with one live CO2 cartridge and one dummy to check pressure retention. No leaks to report three days later.

Just need to tap for the 1/8" NPT fitting and make a check valve that seals the gun as the paintball tank is removed after filling.

More to come...

Making a Bulk-Fill Cap for a Crosman 160--Part 1

Been shooting the Crosman 160 for a week now. I like the rifle, but I'm not a huge fan of using two CO2 cartridges at a time. While I know you can use one full and one empty cartridge in the gun at the same time, I'd prefer to fill the gun from a paintball tank. It'd be a simple matter to order up a bulk-fill cap from either Bryan & Associates or Mac1 as I've done in the past, but I wanted to try my hand at making my own this time around. (And get a break from wood-related projects.)

First a quick word and rant about o-rings and our occasional lapse in mentioning sizes or screwing them up altogether.
As I received the old 160, the o-ring in the cap was a replacement and wasn't exactly CO2 friendly. It's on the right and has swelled with absorbed CO2 gas. The one in the middle has also absorbed some CO2. The o-ring on the far left is new. I kinda hate to measure o-rings from old guns. The one in the gun has obviously been replaced and who's to say that it was the correct size? It sure wasn't the correct material.

OK, so this bulk-fill thingy... It was 9:00 PM and I didn't have a tap to match the Crosman gas tube thread size. So, I looked through my magic parts box and found something useful. Up top is the original cap from the 160. Below is a spare cap from a model 600 semi-auto pistol. I think I can use the retaining ring to mate an otherwise homemade bulk-fill adapter to the 160 rifle. The snap ring has been removed.

Pulled the knurled retaining ring off the cap body. The retaining ring is the key piece as the gas tube thread size is the same on the 600 pistols and 160 rifles.

Gonna start by making a plug to seal the gas tube. Cut a piece of 0.750" tool steel. It's about 1.7" long.

Faced the ends down.

Used a Taig parting tool (Taig part number 1173) to cut a groove 0.100" wide. Slow turning speeds and lots of cutting fluid are needed or the edge of the blade will be destroyed almost instantly and need to be resharpened.

Helps to have minimal blade protrusion from the holder. This stiffens the set up. Flex is not advantageous. OK, the groove diameter is 0.600" at the bottom.

Cut a second groove as a back up. Overkill is OK here. There's plenty of room inside the bore of the gas tube and I need the machining practice.

These will hold the o-rings to seal the 160's gas tube.

Test fit of the o-rings.

Need to make the front of the plug small enough so the retaining ring from the 600 fits over it.



Cleaned up the shoulder that the retaining ring will bear on.

Turned down to 0.579".

Another view.

The long end will be the front.

Ring slip fits. Barely.

Pressure testing. One, two... The first cartridge is new. Second, empty.

One

Two

Plug

Almost seated.

Snugging down the retaining ring seals the deal. Cocking and firing the gun pierces the first cartridge. The replacement plug held pressure! The plug was left long to allow enough room for the 1/8" NPT fitting I have yet to make. In the next few days, I'll need to make that fitting, connect it to the plug and make a simple valve that will seal the cap as the paintball tank is removed after filling.

Crosman 600 Bulk-Fill / Barrel Extension

The Crosman 600 semi-auto has long been considered one of the finest achievements in CO2 airgun development. The 600 uses a small portion of the CO2 gas to cycle and load the gun. It's a true semi-auto, not a revolver like most all the CO2 repeaters on the market today. Ten .22 cal pellets are loaded along the top left side via a built-in spring loaded magazine. (It's actually possible to load one or two additional pellets if they're short--like the Eley Wasp) The CO2 powers a "toggle" action that aligns the forward pellet with the barrel where it's then immediately fired down the barrel. The 600's are typically very pellet sensitive, though the two I have both like .22 cal Eley Wasps, H&N Match Wadcutters, RWS Hobbys, and Crosman Wadcutters. The guns were designed back in the late 1950's and first marketed in 1960. I'm a huge fan of the ray-gun look. The ergonomics, sights and trigger pull are pretty tough to beat. National Match Shooters may note that 600's will often outshoot Ruger Mk 2's offhand at 50 feet on a good day. At the least, it'll be interesting.

The vintage Crosman data indicates that the pistols will fire a 14 grain pellet at 340 fps and 40 shots are available from a 12 gram CO2 cartridge. I've never managed more that 35 shots and I've found the average velocity to actually be about 305 fps. Max velocity with a similar weight pellet is indeed in the quoted 340 range, but the velocity seems to drop pretty quickly into the low 300's and remain there until the gas exhausts.

The stock barrel length is 5". The barrel is easily removed by loosening a single set screw at the rear of the barrel.

Set screw is on left. Top of pistol frame. A 7/64" allen wrench does the trick.

Uh, so what am I doing? Well, a second 600 recently presented itself and I couldn't resist. My single complaint (aside from the gun only holding 10 pellets) is the low shot count per cartridge. The 600 is often--and accurately--described as a "gas hog." One solution to the gas consumption is to increase the on-board CO2 carrying capacity. Many vintage Crosman CO2 guns don't seal the cartridge against the valve. The valve seals the gas tube at one end and the cap seals the other end with the CO2 cartridge in the middle. These guns can benefit from higher shot counts and cheaper CO2 refills by adding an extension tube and a bulk-fill cap. The tube simply adds volume, while the bulk-fill cap allows filling from a paintball tank. Cheap refills and more shooting.

OK, so the bottom 600 has been converted with a bulk-fill adapter and gas tube extension from Bryan & Associates. Tim at Mac1 is another great choice for bulk CO2 parts.

While the gun will work with the 5" barrel in place, it looks absurd with the extension. I had a spare 2260 rifle barrel in my parts stash. One problem: the 2260 barrel diameter is 7/16" or 0.4375". The stock 600 barrel is 0.370" diameter to fit inside the frame.

Eyeballed the length and cut the 2260 barrel down to about 15".

Chucked into the Taig lathe and faced off the cut.

Switched tool bits and cut a tapered lead for the business end of the breech. A high speed (and very light) cut finished the surface like a mirror. This is essentially a "forcing cone" to use a revolver term. The pellet jumps from the toggle into the barrel under rapidly expanding CO2 pressure so I think that is an apt comparison.

Turning down the barrel to 0.370". A live center is being employed to stabilize the end of the barrel at the other end of the bed.

Primarily used a ti-nitride indexable tip to make the cut. Higher speed, heavier cuts.

Finished with a carbide tipped right-hand knife to create a shoulder. The barrel will slide into the 600 frame, seat on the shoulder and lock with the set screw. Length of the 0.370" diameter portion is absolutely critical. Too long, the toggle will hang up on the barrel. Too short, the gap between the toggle to breech is too great allowing CO2 to leak and pellet velocity suffers. Careful and repeated measurements were taken and the length was set at 4.945" from breech end to the shoulder.

Time to cut the muzzle to final length. Another precision cut.

Faced.

Another pic.

Cut a slight recess to protect the crown. Once finished, I cleaned the barrel thoroughly of swarf then lapped the bore with J-B non-embedding paste.

Hand filed a small flat for the set screw.

Sort of like this...

Installed. Barrel ended up at 14.25" long. 9.25" longer than stock.

Couldn't help but make some chronograph comparison notes: We all know that adding barrel length to CO2 guns increases velocity as the CO2 continues to expand behind the pellet.

Stock gun, 12 gram CO2 cartridge, 5" barrel. RWS .22 cal Hobby pellets 11.9 grain:

341* highest velocity
320
314
315
315
310
310
306
309
308
325
318
313
310
307
300
310
300
300
296...

Ending velocity at #33 shot (gas exhaust) was 281 fps.

Bulked gun, with the 14.25" barrel. RWS .22 cal Hobby pellets 11.9 grain:
468
473
472
472
475
478
470
478
478
476
482
480
479
483
480
481
486* highest velocity
477
482
476
482
486
475
466
474
482
481
477
473...

Ending velocity at #69 shot (gas exhaust) was 376 fps--still 35 fps faster than the first and most powerful shot from the 5" barrel.

So, the bulk extension tube essentially doubled the shot count while the 14" barrel added approximately 165+ fps over the stock 5" barrel. 165 additional fps--how often do we see a gain like that?

Downsides? The weight of the tube extension and barrel makes for heavy leverage during offhand shooting. I can do it, and I'm getting better, but it's not the gun for the weak of wrist.

Upsides? You can empty a 10-shot magazine in about 2 seconds and just watch the target bull vaporize. In a couple seconds, a tin can at 50 feet looks like you used a shotgun. --And I don't have to burn through CO2 cartridges anymore.