Rather than explain the background of the XM-248, I will put you in the very capable hands of Weaponsman.com, who has posted an excellent 3-part series on the development leading up to the XM-248, entitled The SAWs that never WAS:
Part 2, the XM-248’s Forerunner, XM235
What I have to add to that information is a copy of the 1979 Ford technical manual on the gun:
Thank you so much for that manual! I’ve been following the posts from WeaponsMan but that tech manual just put me over the top. I’ll have to build one of these or at least repurpose the feed mechanism for a compact belt-fed.
Truly, thanks a million!
“the feed mechanism”
This mechanism reminds me 1930s ShVAK aerial machine gun, as both have rotating sprocket to feed cartridges, however due to different reasons – XM248 mechanism has enhance accuracy of fire, when ShKAS mechanism must be mild enough to not disintegrate cartridges during the process of chambering it, despite the then high rate-of-fire: 1800 rpm
ShKAS was also prone to jamming during tight turns as it was very complex.
Parts 3b, 4 and 5 can be found on WeaponsMan’s blog as well. Makes for good further read.
How does it work Keith, why does it have two gas tubes?
http://weaponsman.com/?p=11661 That’s 3b, kind of a Maxim feed, driven by the lower gas piston as oppose the barrel, so the other one rotates the bolt?
Wow! I didn’t know this manual existed anywhere.
It says it has a hydro pneumatic buffer system on the reverse of the Ford aerospace handout, there’s a link to
a webpage that has info about a hydro pneumatic brake.
In use on an artillery piece, on p65 of the manual it mentions hydraulic fluid as a recommended replacement.
And there’s a mention of a buffer on p55 in the stock. Don’t think it works exactly like as illustrated in the link but it might be along the same lines roughly but with more springs… With like a gas ram, in the stock.
I think the system is essentially a rate reducing mechanism, it doesn’t lock the bolt.
500rpm with not a particularly huge bolt, the layout is also a structural component of design as well as for mechanical function. I like it, can’t say I’ve worked out how it works yet though. Is gas slowing the bolt?
Actually I think it’s distinctive shape led me off on a tangent… Perhaps one rotates the bolt, and the other has something to do with the feeding mech.
Thank you so much!!!
Links are dead 🙁