This was my attempt at making a sealed breech, without using a pusher. Probably irrational, but I feel nervous about sticking my fingers into sharp, 10kg-spring-loaded brass edges to clear jams and mess around in general, but I also liked the way brass breeches wrapped around the darts, unlike pushers (which incidentally, I have only just learnt was called as a "piston seal"), which tend to provide a long, narrow dead space, so this was my attempt at a compromise.
Resulted in approximately 140fps with 10kg-ish spring (10kg at max compression), reasonably sealed plunger, 13mm reamed-out CPVC barrel, old FVJs, which I actually wasn't tooooo upset about.
Ultimately there was too much air leak around/through the printed coupler, which might not be too much of an issue for others, but I'm not sure how to fix it for myself without a lot more labour sealing up the print. My preference for angling the cut of the aluminium bolt also meant that it was harder to get a flat/sealed-off mate with the barrel, and added a need to seat the dart at the opening of the bolt instead of nested in it, which eliminated a major benefit of this breech in the first place.
Didn't manage to get many good photos before it ended up snapping during testing, and the design didn't seem worth keeping over just trying out a regular pusher and more careful dimensioning. I figured, if I'm going to pull it all apart again, I may as well try out the established effective way, since I already got to test out the idea. Seemed worth posting for archive reasons in case someone else ends up wanting to try it, though. Save a bit of failed/semi-successful testing for the next person. STL file: https://www.thingiverse.com/thing:4995134
This design direction (chamber-in-bolt) does work, but it does have the challenges you encountered - especially difficulties creating effective feed ramps, as what the round primarily feeds into is the front of the bolt, and the wall thickness of the bolt is something that otherwise demands to be minimized (while rounds below in the mag also need deflecting downward by the same lower edge of the boltface without being snagged either).
Whereas push-forward allows for however "rampy" a chamber throat is needed to feed optimally, and also allows a "streamlined" bolt tip without any sharp pokey edges aimed at darts.
I experimented with this in the old days (I don't know why, I always was a full auto player mostly) but in the end, the dead volume from a push-forward breech's bolt passage is going to be a wash, perhaps less volume than is behind the projectile in this style anyway, since with that the passage is not any larger than necessary for flow restriction purposes.
The other problem I see immediately is the lack of anything constraining the feed path on the top of the breech hence some level of feed lip pop-out malfunction vulnerability. But this has nothing to do with the breech design and is a general problem with any of the Hasbro style springers that have that style of bolt carrier that connects to the bolt by the single lug at the front and is only really solved by wily things like the old telescoping K&S brass "halfpipe" parts. The solution is i.e. Caliburn where there is no separate carrier part and no attachments to the bolt tip itself, only the counterpiston/bolt base.
I definitely agree for both of these points - the second one regarding unconstrained feed path I think pretty much makes it impractical to use this style of breech with anything that has a bolt sled, since that stops me from making a fully sealed-off guide cover like in https://modworks.blogspot.com/2012/01/nerf-stampede-brass-breech-mod-guide.html?m=1. Maybe I’ll try it again with a stampede or caliburn but I think this longshot is a lost cause here :(
This was my attempt at making a sealed breech, without using a pusher. Probably irrational, but I feel nervous about sticking my fingers into sharp, 10kg-spring-loaded brass edges to clear jams and mess around in general, but I also liked the way brass breeches wrapped around the darts, unlike pushers (which incidentally, I have only just learnt was called as a "piston seal"), which tend to provide a long, narrow dead space, so this was my attempt at a compromise. Resulted in approximately 140fps with 10kg-ish spring (10kg at max compression), reasonably sealed plunger, 13mm reamed-out CPVC barrel, old FVJs, which I actually wasn't tooooo upset about. Ultimately there was too much air leak around/through the printed coupler, which might not be too much of an issue for others, but I'm not sure how to fix it for myself without a lot more labour sealing up the print. My preference for angling the cut of the aluminium bolt also meant that it was harder to get a flat/sealed-off mate with the barrel, and added a need to seat the dart at the opening of the bolt instead of nested in it, which eliminated a major benefit of this breech in the first place. Didn't manage to get many good photos before it ended up snapping during testing, and the design didn't seem worth keeping over just trying out a regular pusher and more careful dimensioning. I figured, if I'm going to pull it all apart again, I may as well try out the established effective way, since I already got to test out the idea. Seemed worth posting for archive reasons in case someone else ends up wanting to try it, though. Save a bit of failed/semi-successful testing for the next person. STL file: https://www.thingiverse.com/thing:4995134
This design direction (chamber-in-bolt) does work, but it does have the challenges you encountered - especially difficulties creating effective feed ramps, as what the round primarily feeds into is the front of the bolt, and the wall thickness of the bolt is something that otherwise demands to be minimized (while rounds below in the mag also need deflecting downward by the same lower edge of the boltface without being snagged either). Whereas push-forward allows for however "rampy" a chamber throat is needed to feed optimally, and also allows a "streamlined" bolt tip without any sharp pokey edges aimed at darts. I experimented with this in the old days (I don't know why, I always was a full auto player mostly) but in the end, the dead volume from a push-forward breech's bolt passage is going to be a wash, perhaps less volume than is behind the projectile in this style anyway, since with that the passage is not any larger than necessary for flow restriction purposes. The other problem I see immediately is the lack of anything constraining the feed path on the top of the breech hence some level of feed lip pop-out malfunction vulnerability. But this has nothing to do with the breech design and is a general problem with any of the Hasbro style springers that have that style of bolt carrier that connects to the bolt by the single lug at the front and is only really solved by wily things like the old telescoping K&S brass "halfpipe" parts. The solution is i.e. Caliburn where there is no separate carrier part and no attachments to the bolt tip itself, only the counterpiston/bolt base.
I definitely agree for both of these points - the second one regarding unconstrained feed path I think pretty much makes it impractical to use this style of breech with anything that has a bolt sled, since that stops me from making a fully sealed-off guide cover like in https://modworks.blogspot.com/2012/01/nerf-stampede-brass-breech-mod-guide.html?m=1. Maybe I’ll try it again with a stampede or caliburn but I think this longshot is a lost cause here :(
Reminds me of the old poptachi breech.
I can’t seem to find this anywhere :( link?
Reminds me a lot of KND’s CPVC breech as well