Spacehog 2,218 Posted January 19, 2017 Report Share Posted January 19, 2017 (edited) I am finishing up a SBR build with a Mini Draco as the starting point. I have come up with a way to put a traditional looking AK handguard on the front but have a couple of concerns and need some advice from those here that build or manufacture. Here is the gun with the handguards I want to mount: I want to use a cut down regular sized gas tube with the upper hand guard mounting bracket. I have cut one down and it fits well on the Mini with no play: My question comes in with the comparison of the standard gas tube and the factory mini Draco gas tube. Although they have the same dimensions externally, the mini Draco tube is substantially heavier and thicker. It is chrome lined, and this may be the reason it feels so much more heavy duty. Pics below: The standard is on the left and the Factory on the right. Would there be any reason for the Mini Draco with its short piston rod to need a heavier tube? It has always been my understanding that the tube does nothing more than act as a guide for the piston. I understand that the chrome lining would help with longevity when firing corrosive ammo, but apart from that any other reasons for the beefier tube? Correction, the Mini gas tube may not be chrome lined. Upon closer inspection, it may be that it is just polished from the piston movement. Either way, it is definitely heavier duty. Edited January 19, 2017 by Spacehog Quote Link to post Share on other sites
gunfun 3,931 Posted January 19, 2017 Report Share Posted January 19, 2017 Is the i.d. the same? Thicker walls = smaller i.d. you might need to crimp it a little to have it guide the piston along the same track. Or instead, sleeve a smaller tube inside. Quote Link to post Share on other sites
Spacehog 2,218 Posted January 19, 2017 Author Report Share Posted January 19, 2017 (edited) Both are 20mm +\- .05 mm OD. The interior of the Mini tube is smaller. With the larger ID replacement tube installed, I have hand cycled the piston rod numerous times. There does not seem to be any excess play and everything is smooth. Edited January 19, 2017 by Spacehog Quote Link to post Share on other sites
gunfun 3,931 Posted January 19, 2017 Report Share Posted January 19, 2017 Why not try it and see? It sounds fine to me. Cover it all in ink and look for any signs of wierd wear. 1 Quote Link to post Share on other sites
Spacehog 2,218 Posted January 19, 2017 Author Report Share Posted January 19, 2017 At this point that is the plan. Just don't want it to go boom in places it's not supposed to. Quote Link to post Share on other sites
David Mark 2,452 Posted January 20, 2017 Report Share Posted January 20, 2017 Engineers rarely "overbuild" a part especially those working off government contracts. If the original gas tube has a thicker wall it's probably for a reason. Perhaps more pressure to deal with due to the shorter barrel? Looking forward to hearing the results of your tests, Just be careful. 1 Quote Link to post Share on other sites
Sim_Player 1,939 Posted January 20, 2017 Report Share Posted January 20, 2017 (edited) Maybe they were concerned that a factory thin-walled tube would split when they tried shaping it. There is a lot of slop in standard gas tubes, even with the star-shaped crimping. I see they made the I.D. much smaller on the other. Are you going to crimp the walls? Edited January 20, 2017 by Sim_Player Quote Link to post Share on other sites
Spacehog 2,218 Posted January 20, 2017 Author Report Share Posted January 20, 2017 Maybe they were concerned that a factory thin-walled tube would split when they tried shaping it. There is a lot of slop in standard gas tubes, even with the star-shaped crimping. I see they made the I.D. much smaller on the other. Are you going to crimp the walls? Due to the short length, I was not planning on it. If I have to, I will sleeve it as GunFun suggested, but I still question how much pressure the gas tube has to endure as all of them leak like a sieve when fired, ported or not. Quote Link to post Share on other sites
Sim_Player 1,939 Posted January 20, 2017 Report Share Posted January 20, 2017 (edited) Is the other one vented? My only concern would be over-gassing the bolt carrier. I dought it would actually explode. Edited January 20, 2017 by Sim_Player Quote Link to post Share on other sites
Spacehog 2,218 Posted January 20, 2017 Author Report Share Posted January 20, 2017 Neither are vented at the gas block. Both naturally vent rearward where they seat at the front trunnion. I am beginning to suspect that the ID is smaller with the mini to actually increase pressure on the piston to help with cycling on the 7.5" barrel. If this is true, then the larger tube may allow too much gas to bypass the piston resulting in poor cycling. Only a trip to the property is going to answer that question. Quote Link to post Share on other sites
Spacehog 2,218 Posted January 20, 2017 Author Report Share Posted January 20, 2017 Update: It is there to tighten up bolt carrier travel. There is just enough play with the larger tube where I can hang the bolt carrier if I don't rack back and let it fly. Probably wouldn't be an issue with live fire, but would increase wear and tear on the carrier. Play goes away with the factory tube. Back to the drawing board. Quote Link to post Share on other sites
G O B 3,516 Posted January 20, 2017 Report Share Posted January 20, 2017 The shorter tube will have a larger force imparted to it. The longer rod gives the full length tube considerably less side force. Basic lever. Quote Link to post Share on other sites
gunfun 3,931 Posted January 20, 2017 Report Share Posted January 20, 2017 brass sleeve. Adds lubricity to the system while you align it. 1 Quote Link to post Share on other sites
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