3D printed a new QB78 breech. Multi shot coming soon

[AustinBinTX]

I’ve been messing around with making breeches for QBs for a while now, but I can’t seem to get enough time to devote to running the mill to make enough for all of my rifles.

I have a couple decent 3D printers, and I’m decent with CAD, so I figured why not give it a shot. It’s not like the QB breech sees any real pressure. It’s all contained in the transfer port, and that can be easily reinforced.

My ultimate goal is to make multi shot breeches for all of my QBs that are tall enough that I can shroud the rifles for backyard pesting. I don’t need any more power than CO2 provides, but I would like to be as stealthy as possible.

I printed a few tests over the last few days to test dimensions and fit. I ended up with a perfect fit on the fifth print. At 4.5 hours a print, it was definitely a sigh of relief.

A modified barrel band had to be printed as well to keep any extra stress off of the breech.

Everything went together perfectly and worked perfectly with no leaks. The dovetail holds the scope fine, and I had no issues zeroing. I’ll have to continue testing to see how well it will hold zero though. I imagine since I’m printing from PETG, it should be fine.

I plan on testing this single shot version for a couple weeks. If everything works as I’m expecting, I’ll chuck up the barrel and turn it down to fit a Marauder magazine. I’ll turn down the bolt if needed, and I’ll print off the multishot version of the breech I drew up. I’ll slap it all together, and have myself a poor mans multishot QB.

[mackeral5]

Very cool!

[Rob M]

very nice , what % infill?? you could also extend the front for more barrel interaction

[AustinBinTX]

very nice , what % infill?? you could also extend the front for more barrel interaction

Rob,

I’m printing them with 0.6mm nozzles at 0.3mm layer height. They have 4.2mm walls with 50% infill. I’m printing with PETG at 250C and a bed temp of 90C. Everything is fused together almost perfectly. No flex whatsoever.

It took a bench vise, a 3# sledge, and some serious muscle to crack the first test ones.

As far as extending the front, I had printed one tester that had an extended front, but it didn’t really make any difference. A good fitting barrel band did more for rigidity than extending the breech. A 15mm barrel hanging off the end of the metal QB breech will flex down just as much as these, so I’m not overly concerned about it. I would however, like to protect it from any extra added stress caused by hitting an unsupported barrel on things.

[AustinBinTX]

Rob,

One thing I forgot to add was that the multishot version may need to be printed with 100% infill.

The cutout for the Marauder magazine will make the breech more prone to flexing, so it would definitely benefit from all the extra material that can be fit in there.

I’m off work tomorrow. I may start the multishot breech print(5.5ish hrs) and put the collet chuck on the lathe so I can turn the barrel and bolt down for the Marauder mag.

[Rob M]

awesome.. ive seen this attempted before but in lackluster fashion.. You may have cornered an entire market here if you spread the knowledge to discoveries and so on..

[AustinBinTX]

awesome.. ive seen this attempted before but in lackluster fashion.. You may have cornered an entire market here if you spread the knowledge to discoveries and so on..

Rob,

I know there are some downsides to printing parts like this.

Longevity is one. A metal bolt cycling enough times will eventually wear the fused material.

Strength is probably the biggest concern for me. PETG is strong, but PEEK would be a much better option, that is, until you look at the cost for a half kilo of PEEK filament and the requirements to print it.

I’ll probably make a few of these to put on CO2 only rifles. If anyone wanted to test one of the single shot breeches, have a couple extra. I could print off matching barrel bands for whichever version QB you had.

[Rob M]

never printed in petg... I wonder if nylon would be better for surface friction durability ??  Youd need a volunteer who puts down a tin a week in a QB.. this way you get accelerated wear results. If i had to guess id think it would last quite some time so long as the bolt is polished etc

[mackeral5]

Rob,

One thing I forgot to add was that the multishot version may need to be printed with 100% infill.

The cutout for the Marauder magazine will make the breech more prone to flexing, so it would definitely benefit from all the extra material that can be fit in there.

I’m off work tomorrow. I may start the multishot breech print(5.5ish hrs) and put the collet chuck on the lathe so I can turn the barrel and bolt down for the Marauder mag.

The mag cutout flexes in the aluminum versions, but not much of an issue <25fpe.  For my lower powered repeaters I never ran a barrel band.  Once you get up in the 25fpe+ range recoil/barrel harmonics come into play and breech flex is a real problem..

[AustinBinTX]

The mag cutout flexes in the aluminum versions, but not much of an issue <25fpe.  For my lower powered repeaters I never ran a barrel band.  Once you get up in the 25fpe+ range recoil/barrel harmonics come into play and breech flex is a real problem..
[/quote]

I may try printing from some CF reinforced Polycarbonate I have here. It’s by far the strongest filament my little Prusas can print. I just need to work on a better enclosure to keep the ambient temp up high enough to prevent warping.

Since the aluminum breeches flex as well, I’ll just roll with it. I’m sure if it would help, I could print the breeches off with internal slots on each side for an aluminum dowel rod on that would run the entire of the breech to stiffen it up more.

Too many ideas and not enough time to work through them all it feels like.

[larspawn]

I am seriously impressed and amazed at what technology has brought us.  Couldn't you solve the durability problem by sleaving the bolt probe area?

[Chako]

Even if the 3D print doesn't work out, you would still have the 3d file for the multishot breech that could be cut out of metal. As far as I can tell, nobody sells them new anymore.

[AustinBinTX]

I am seriously impressed and amazed at what technology has brought us.  Couldn't you solve the durability problem by sleaving the bolt probe area?

Sleeving it would definitely work. I had given thought to actually using the entire original QB breech as the sleeve, but I decided against it. Now that you asked the question though, I could turn the entire steel QB breech down in the lathe and press a lightweight version of it in a modified printed breech. It would save me the time of having to do all of the machine work from scratch. I don’t believe it would take more than 5 minutes to modify my CAD file for a sleeve either.

Thank you all for the questions, suggestions, and insight. Please keep them coming.

[AustinBinTX]

Even if the 3D print doesn't work out, you would still have the 3d file for the multishot breech that could be cut out of metal. As far as I can tell, nobody sells them new anymore.

True, but I don’t see myself replacing my manual mill with a CNC mill, and I really don’t think I could get a good enough deal to have a run of them machined somewhere.

[mackeral5]

The mag cutout flexes in the aluminum versions, but not much of an issue <25fpe.  For my lower powered repeaters I never ran a barrel band.  Once you get up in the 25fpe+ range recoil/barrel harmonics come into play and breech flex is a real problem..

I may try printing from some CF reinforced Polycarbonate I have here. It’s by far the strongest filament my little Prusas can print. I just need to work on a better enclosure to keep the ambient temp up high enough to prevent warping.

Since the aluminum breeches flex as well, I’ll just roll with it. I’m sure if it would help, I could print the breeches off with internal slots on each side for an aluminum dowel rod on that would run the entire of the breech to stiffen it up more.

Too many ideas and not enough time to work through them all it feels like.
[/quote]

Or design so that the mag inserts into a mag well vs a slot----bridge the dovetail gap.  This would mean a slightly higher dovetail, but I think the rigidity would be worth it.

[AustinBinTX]

The mag cutout flexes in the aluminum versions, but not much of an issue <25fpe.  For my lower powered repeaters I never ran a barrel band.  Once you get up in the 25fpe+ range recoil/barrel harmonics come into play and breech flex is a real problem..

I may try printing from some CF reinforced Polycarbonate I have here. It’s by far the strongest filament my little Prusas can print. I just need to work on a better enclosure to keep the ambient temp up high enough to prevent warping.

Since the aluminum breeches flex as well, I’ll just roll with it. I’m sure if it would help, I could print the breeches off with internal slots on each side for an aluminum dowel rod on that would run the entire of the breech to stiffen it up more.

Too many ideas and not enough time to work through them all it feels like.

Or design so that the mag inserts into a mag well vs a slot----bridge the dovetail gap.  This would mean a slightly higher dovetail, but I think the rigidity would be worth it.
[/quote]

That’s a great idea for sure. I wonder about scope height with it though. I’ll have to draw everything up and see what it ends up looking like.

I’ll probably print the original design I have for the multishot breech tonight. It’ll give me time to machine my backplate for my new Pratt and Burnerd KC25 multisize collet chuck. I’ve been like a kid on Christmas morning waiting to use it, but I keep getting side tracked with the 3D printers.

[AustinBinTX]

Whoops, I’ll print the design I have in hand, and then make changes as needed.

It’s hard for me to visualize revisions without having a prior version or two in my hand.

[Rob M]

Whoops, I’ll print the design I have in hand, and then make changes as needed.

It’s hard for me to visualize revisions without having a prior version or two in my hand.

at co2 pressures i think it will be fine .. especially with the internal sleeve suggested above. A small section of brass tube inserted where the bolt rides would cure wear and friction at once.

[rsterne]

Definitely need a good barrel band to prevent the barrel from leveraging on the front of the receiver and breaking it off, IMO.... I can't imagine any plastic not reinforced with continuous carbon fibres (which you can't print) being anything like as stiff as aluminum, let alone the original, which was steel.... I have always hated the short barrel inset in QBs.... The barrel wobbles around something fierce unless additional setscrews are added from the sides at the back of the loading tray....

Don't forget that the forward force on the barrel and the rearwards force on the bolt both are equal to the pressure times the area of the front of the bolt.... For a .22 cal bolt, that is 0.038 sq.in., so at 850 psi it is 32 lbs. but at 2000 psi it is 80 lbs.... Increase it to a .25 cal (0.049 sq.in) and it goes up to about 100 lbs.... If you are holding the barrel in with a setscrew, consider the loads on the plastic at the threaded hole....

Bob

[Insanity]

Intrested in durability.