Cutting down plastic or synthetic stock.

Not sure what the actual problem was that you encountered. Maybe post pictures. 
Your saying even cutting a plastic stock by hand with a hack saw. You still melted the plastic? 

Slow even cut with a carpenter’s crosscut saw. Fine tooth saw blades gum up and start melting plastic quickly. Final shaping with course sandpaper and final with a sharp scraper.

Different manufacturers use different materials in their synthetic stocks which is why they are grouped as synthetic and not plastic or glass filled nylon. From what you are saying you are using a saw that is too fine. I use a bandsaw and wrap the stock with a few wraps of masking tape which allows me to see the line and the fence on the saw allows me to set depth of cut. If you don't have a bandsaw with a coarse blade you might be able to use a crosscut or rip saw. Again my suggestion is based on your experience as described because a stock that is plastic will react differently than a glass filled nylon stock when cut.

Try finding an older Disco with the wooden stock. Another option is the Marauder pistol. That stock is very easy to cut down and a custom cheek piece is easily made with a length of dowel and a couple of machine screws. Another plus with the pistol is the size and weight will probably work out better.

Dremel with cutting wheel. Not the one that attaches by the screw. You need the one that has its on holder and is a hard disk. For inlay I use an end grinder with a bur bit. Yes it will melt the material but the excess is easy to remove and finish out. Most of the syn will buff out and blend. I use this method all the time on stock conversions.

But as said not all syn stocks are made out of the same material but this method should work an all. JMO

Back when I ran my custom shops, I used to bend/cut/mold/fuse plastics all the time.. 
As mentioned above, rotary tools with thin cut-off-disks worked the best. 

I used a Foredom, but a Dremel would work just the same..

Figure out where you'd like to make your cut, and rather than draw or tape lines, 
I recommend using something sharp to actually scribe the line. 
That way, the scribe indentation will help guide the disk.. 

Since the friction will melt the material, you'll want to do a few shallower passes 
rather than just try to plunge and do one cut..

This helps cut, while cooling as you go - re-solidifying the plastic.. 
After a few passes, you'll be good to go.
As for any mess.. the extra blobs of melted plastic usually chip right off..
As for finishing, you can use emery or sanding papers, and water can help.. 
Just don't sand like a maniac or with too much pressure or you'll build up friction/heat. 
Let the tool / paper do it's job..

There's also a method of using concentrated heat [small torch or heat-gun]
to shape/melt the material smooth[er] again, but if not comfortable, maybe don't try that.

This sounds like a really good idea, and fun project! 

 

Sam -

PS: Some hobby shops sell plastic sheets in different sizes and thicknesses.. 
For the end of the stock, that stuff could be traced to form the 'end cap', 
then adhered with something like CA-glue - The thick / gap-filling version. 
If ya did a few layers to build up material thickness, there'd be enough meat 
to sink a few screws into to hold a rubber pad. Or just get creative.

This is what I came up with for my grandkids after that do well with their BB gun. I found the plastic stock was too long for the kids and found this wood butt stock made for crosman 2200 through2400 series fit the Benajmin Prod and shorten the grip now all but two can shoulder this gun and they shoot it very well. Oh I have 5 grandkids and two are 2 years old lol i