Monday, March 18, 2013

Crosman MK2 Modifications -- Part 2

I was going to rebarrel this Crosman MK2 to either .20 or .22 cal, but since someone already took the time to machine a really nice .177 cal loading bolt, I sourced a 24" Crosman 1760 barrel instead.

The stock MK2 barrel is just shy of 8" OAL. 

Cut the new barrel off somewhere around 10.3" long.  Assuming  that the final length will be pretty close to 10" after threading, facing and crowning.

Measured the major diameter of the thread--the barrel needs 3/8"-24 threads at each end.

Chucked the new barrel in the Taig and turned the end down to 0.369"  Cut a slight taper at the very end to make the threading die easier to start.

With a die holder mounted to the tailstock, I started the thread. 

Eventually, the barrel slipped in the chuck, so it was transferred to rubberized jaws in a bench vise and the thread was finished.

Back in the Taig to face then cut a 0.370" shoulder behind the thread.   This will be the breech end of the barrel.  I'll be working breech to muzzle.


A little bit of measuring gave the location for the CO2 transfer hole in the breech-end of the barrel.  Clamped the barrel in a v-block (this proved to be a mistake) and lined up to drill the hole.


Drilled with a #28.   Removed the barrel and immediately realized I had no way to line up this hole with the transfer port in the gun.  The original barrel features a dimple for the barrel setscrew opposite from the CO2 hole.  The dimple allows indication by alignment with the threaded hole in the gun's receiver before installing the setscrew. 

Mounted the barrel in a 5C collet and a square collet block.  Took a couple minutes to orient the hole so it faced straight up and was centered under the mill spindle.   Positioned a work stop against the collet block.  Flipped the collet block over and butted against the work stop.  Drilled a shallow hole with a spotting drill.

Began the cut for the pellet leade with a small tapered burr.

Knew it wasn't going to be deep enough, so I started looking around for something to use.  Remembered I had a set of tapered pin reamers that are likely a few decades older than I am.

The #2 was a likely looking candidate.  Slow speed and lots of cutting oil. Lot of test fitting followed by several small pieces of emery cloth backed with a small brass rod to complete the polishing.

Another test fit.

 Bolt rotates closed. 

Not shown.  Threaded the muzzle end of things.


Recessed, crowned and polished to 500 grit.

After pondering the various sizes of metal stock in the basement and weighing the pros and cons of steel vs. aluminum,  I finally settled on a 7/8" DOM steel tube to sleeve up the barrel. 

Out in the garage, the tube was chucked in the Shopmaster lathe and the end was turned down to 0.755" OD.  This will slip fit into the breech of the receiver.

Cut the tube 7.125" from the step at the shoulder to the muzzle.

Test fit.  So far, so good.  In the next couple days, I'll make a nut for the muzzle.  It will tension the barrel as well as secure the 7/8" barrel sleeve.  Then I'll need to make a plug to act as a base for the hammer spring and fit (or make) a new front sight.

Please check back soon.