Crosm 1377 conversion to .22 carbine.

[Multigunner]

Well I'm all but finished with this conversion, Still some work to do but I mounted a new .22 receiver with bolt and a .2260 .22 24" barrel.

I also obtained the proper end cap with band for the conversion. I found I had to cut away part of the front surface of the pump arm grips in order for it to clear the longer extension of the end cap, otherwise I couldn't get a full stroke when pumping.

I'd intended to cut the 2260 barrel down to 14 inches, but I like it at full length for now.
They say that 14.5 inches is the maximum length for full acceleration of the .22 pellet when using a stock valve body and pump. There may be a slight loss in velocity due to friction when the barrel is longer than that.
The 24" barrel is a very quiet shooter, no need for a muzzle brake or suppressor.
Velocity and penetration using the standard 14.3 gr hollow point is outstanding.

I haven't mounted a front sight yet. I intended to file one to suit my needs from brass. I've made front sights for Cap&Ball pistols from brass in the past.

Main reason I bought this barrel to begin with was because I planned to make a new bore liner for my 38T, I already have the steel .22 cylinder for that project.

After I borrow a chronograph to compare the velocity of the carbine as it is I'll be cutting off a six inch section for the liner leaving an 18" barrel. If I like it like that I'll leave it at 18". If I lose much velocity that way I'll probably get another 24" barrel next time I find them on sale. This barrel was pretty reasonably priced at 25 bucks. I also found a stock on sale as well. I'd been offered an older production stock but found those won't fit the late model frame without modification. The old 1322 frame has one locator pin per side while the late model has two.

I major factor in deciding on the conversion was that .177 pellets are just too small to get into the chamber easily.
Striking power and penetration is so much greater in .22 that its unlikely that I'll ever switch back.

I may not ever scope this carbine, but if I do I have a few ideas for a home made base.

The steel breech conversions are the best for mounting a substantial scope, but I can't see that the expense is justified for my purposes.

[Tracy]

I'm pretty sure that what you heard about 14.5" being the max length for velocity, is not true.
I know for a fact that it is not true with co2, nor PCP. And what is a pumper but a PCP with its own, onboard air source?

With co2 and PCP guns, velocity is still increasing beyond 24 inches. It's possible that a given 13xx could be at such a low state of tune that it sees no gains beyond 14.5 inches of barrel length. But if so, it would be no problem to tune it to effectively use a longer barrel.

[Multigunner]

"But if so, it would be no problem to tune it to effectively use a longer barrel. "

No technical problem but the parts would add a bit to the expenses.

From what I've found on this sort of conversion its a matter of total volume of the valve body.
A valve body with more internal volume can, at the same pressure level in psi, drive the pellet to higher velocities in a longer barrel. Its a matter of length of travel vs total volume of air.
To some extent the .22 RF rifles ,with long barrels, when using shorts or CB caps display the same phenomena.

Since the available air has expanded to its limit there's little to no muzzle blast. What air is left in the bore isn't moving fast enough to upset the air around it.

I don't intend to hotrod this particular rifle. I'm well pleased with the present velocity and penetration.

The information on the 14.5" barrel being the max for efficiency with the stock valve came from Crosman by way of the Pyramid Air Blog site.
There also ran chrono tests on velocity per number of pumps and found that 11 pumps are the max for highest velocity when using the standard valve and striker springs. Past 11 pumps requires a stronger strike to open the exhaust valve fully.
I intend to go no shorter than 18" in any case.
All the power mods done to these guns are intended to overcome those factory limitations.

If when I fit a front sight the rifle shows itself to be exceptionally accurate with the 24" barrel I probably won't cut it down at all.

With the 24" barrel the balance point is right in front of the trigger guard, perfect for long term carrying.

I plan to make a sturdy wooden pump grip later on. The hollow plastic factory grip is way too fragile. With it's extra weight up front the removal of six inches of barrel should put the balance point back in the same spot.

[jh45gun]

Crosman Custom shop offers an 18 inch barrel for their 1300 kt 22. I went with the 14.5. I wanted a more compact carbine with some barrel length.

[Tracy]

Good comments. It costs very little to improve the tune, though. You can drill and tap the rear cap for a power adjuster in the form of a 10-32 screw, remove about half of the check valve for more volume, remove a bit of the nose of the valve and shim the piston to reduce headspace and hence improve efficiency and power, and stuff the piston. All for very little money.
But you said you don't want to hotrod it, and I can understand that. I've hotrodded lots of my airguns, but I have a 1377 that is stone stock and is one of my favorite plinkers just the way it is.

BTW, Baker Airguns has late Benji 342 pump handles, both checkered and smooth, for $15. They are walnut. That might be a good handle to fit to your carbine.

[Multigunner]

"Baker Airguns has late Benji 342 pump handles"

Thanks I'll look into those.
I have some well seasoned walnut planks that I'd probably use if I make my own. I do my own checkering for handgun grips, though I haven't checkered a grip in many years.

I made a temporary front sight by simply using the old nose cap with sight , wrapping some tape around the muzzle for a jam tight fit.
The blade height seems to be close enough for now.
I just tried a shot at a stew can offhand at twenty yards and hit it on the first shot. I'll fire for group sizes later today.
This will do for test firing purposes.

A small washer behind the rear end of the spring would increase preload and striking speed. The spring of my gun seems to be a bit heavier than I'd have expected anyway.

When I get the use of the chronograph I'll be able to tell if my gun benefits from extra pumps. I'll record FPS at ten and eleven pumps and then twelve. If there's a velocity increase at twelve pumps I'll continue increasing the number of pump strokes till it maxs out.

[Tracy]

The velocity of mine increases up to about 12 pumps. Much above that and you need to stuff the piston, as it begins to distort under the pressure.

[Multigunner]

"as it begins to distort under the pressure. "

What form of distortion takes place?

So far I've found a remarkable increase in striking power at 15 pumps compared to 12 pumps.

I'm measuring this by how deep and how wide a dent the pellets make in the side of a empty camp gas propane canister, and how deep the pellets go into soft pine. Chrono tests will give me a better idea of what's going on.

There's no retained air in the valve after a shot.
The spring of my gun seemed uncommonly stiff to cock when I first got it. perhaps its stronger and snappier than the factory specs would allow for. A matter of manufacturing variation I suppose.
In any case the striker hits hard enough that the valve exhausts fully at 15 pumps. So far there's been no sign of seepage if left fully charged for an hour or so at a time.

[Tracy]

The piston is plastic, and shortens a bit when you pump much over 10 pumps. This increases the headspace between piston head and check valve, so pumping becomes less efficient. It helps to epoxy some 3/16" aluminum rods into the spaces between the ribs. It's even better (but more expensive) to replace the piston with a metal one.

[Multigunner]

"It's even better (but more expensive) to replace the piston with a metal one. "

I'll do that if I notice any problem with the present piston arm.

I did not intend to hot rod this particular conversion, but I might spend a bit on it later on if I decide to keep it, and so far its been a real keeper.

I've considered ordering a Custom Shop 1300 KT with a few bells and whistles and giving this conversion to a young nephew who only this Christmas got his first Daisy BB gun from his dad. I don't think he is quite old enough or big enough to handle a pump up rifle at full power just yet, but maybe in a few more years.

The dinky stock peep sight and plastic receiver make obtaining a fine zero a bit chancy but I keep all my shots in a 2" square post it at 25 yards without a bench rest and only the peep sight . The first few shots will print dead center in a Nickel sized group ( I actually make bullseyes for my targets by tracing around a nickel then filling it in with a sharpie, the front sight is wider than the bullseye at that range) , but then the effort of pumping it up between shots begins to show itself and groups get wider with more than a few called flyers.
I'm sure that from a bench rest the rifle could print dime size groups all day long. If I were rich I guess I could hire someone to pump the rifle up for me between shots.

Snap shooting is where this rifle shines. I feel its already in shape to be an effective small game rifle at 25 yards perhaps further. Headshots on squirrels at 25 yards when I'm not fatigued would be no problemo.

If the rifle chronographs anywhere near 600 FPS at 15 pumps in its present state I figure it should also be good enough for body shots on running cotton tails, which don't seem to require much to kill , the local variety at least. I've seen these die instantly from a single no.6 shot in a hind leg. I guess their little hearts can't handle much traumatic shock when geared up for flight.

Speaking of Wabbits I've found that Tennessee has its own species that people seldom see. its a woods dwelling creature, tiny and well camouflaged. I won't be shooting any of those, I expect they are fairly rare if not endangered.
Twice I've found huge Walloon rabbits in my yard, strayed from a local breeder's pens. These buggers are enormous. One let me pick it up but then it freaked and ripped through my field jacket and shirt with its hind claws as it kicked its way out of my grasp. I had several deep broad 6" scratches on my ribs and abdomen that bled quite a bit. They have hind legs with plenty of muscle. They can reach over 50 pounds, these were probably 35 pounds or so. One ( the one that kicked me) was a beautiful sable while the other was snow white with red eyes.