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Posted: 5/29/2015 12:13:51 AM EDT
| Hi All-I have only built two basic AR's, but have been wanting to build a mid length gas dissy for a year or so. I have all the parts but a barrel and fab. My question centers around pinning the fsb at rifle length. On ADCO's FAQ page they say pinning at rifle length will not work for most barrels since they are not .750 at that length. So I am a bit confused before buying a barrel to send to them with the proper fsb . Would any m4 or gov profile barrel which is ,750 at the gas port work? I am making the assumption they are .750 at rifle length for fsb. Please help so I can get the barel and complete my dissy. I plan on finding a 16" chf fnor DD barrel. But if you guys have one you really liked for your dissy I could be swayed. Thanks |
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Quoted: Hi All-I have only built two basic AR's, but have been wanting to build a mid length gas dissy for a year or so. I have all the parts but a barrel and fab. My question centers around pinning the fsb at rifle length. On ADCO's FAQ page they say pinning at rifle length will not work for most barrels since they are not .750 at that length. So I am a bit confused before buying a barrel to send to them with the proper fsb . Would any m4 or gov profile barrel which is ,750 at the gas port work? I am making the assumption they are .750 at rifle length for fsb. Please help so I can get the barel and complete my dissy. I plan on finding a 16" chf fnor DD barrel. But if you guys have one you really liked for your dissy I could be swayed. Thanks Heres a great deal on a 16" dissipator chrome lined midlength gas system http://www.ar15.com/forums/t_7_159/1426949_wts_spikes_barrels_added_more_dissy_barrels.html |
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Do you understand what a dissipator is? Your post seems like you don't. A dissipator has a functional low profile gas block at carbine or mid length, and a standard FSB at rifle length that it only used for the sights. It's not just a matter of pinning a FSB, the barrel has to be contoured to fit the FSB. What ADCO is saying is correct, the .750" diameter tapers off to around .735" forward of the gas port so a standard FSB wouldn't fit properly.
Have you contacted ADCO? You're probably better off finding a 20" rifle barrel, having them cut it down to 16" and relocate the gas port to the middy location. Is there any specific reason you're stuck on this configuration? It seems like more trouble than it's worth. A quick google search shows that even the PSA Midlength dissy uppers had issues. ETA: that barrel posted above is pretty much exactly what you're looking for, just buy that and be done with it |
| Yes I know what a dissy is. I already have my low profile block and mid length tube, my question was solely about fsb and barrel diameter at rifle length. I assumed folks had learned in their builds and was seeing how they solved that issue. I was hoping to buy a barrel I liked and then to send to ADCO for fab pinning and just wanted to understand better. |
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Quoted:
Yes I know what a dissy is. I already have my low profile block and mid length tube, my question was solely about fsb and barrel diameter at rifle length. I assumed folks had learned in their builds and was seeing how they solved that issue. I was hoping to buy a barrel I liked and then to send to ADCO for fab pinning and just wanted to understand better. Bushmaster was one of the first Companies to produce a Dissy that was reliable, PSA also makes a good one as does Spikes. I have had 2 of the Bushmaster Dissys and both ran like champs, I still have one that I don't plan on parting with because it was purchased when the company was still in Windham and I refuse to do business with the new coompany that calls themselves Bushmaster, they share parts with DPMS and the barrels are different, and marked BFI instead of B MP. One look at the barrel though and you can tell even before looking at the barrel stamp. I don't know if Windham Weaponry makes a Sissy barrel, but I would run one as they are the same Company/Guys/Machinery as the pre sell out Bushmaster. |
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Quoted: I don't know if Windham Weaponry makes a Sissy barrel, but I would run one as they are the same Company/Guys/Machinery as the pre sell out Bushmaster. Quoted: Quoted: Yes I know what a dissy is. I already have my low profile block and mid length tube, my question was solely about fsb and barrel diameter at rifle length. I assumed folks had learned in their builds and was seeing how they solved that issue. I was hoping to buy a barrel I liked and then to send to ADCO for fab pinning and just wanted to understand better. I don't know if Windham Weaponry makes a Sissy barrel, but I would run one as they are the same Company/Guys/Machinery as the pre sell out Bushmaster. http://www.ar15.com/forums/t_2_489/267443_.html&page=1&sr=0 Like this?  |
| Most barrels are turned down slightly less than .750 in front of the FSB (ETA to clarify: the gas port area where the maker INTENDED for a FSB to go). I am building a dissy and I went carbine length because I don't care and I found a great dea;l on a M4 barrel from classic arms (like $65 on black friday!) It turned out the BCA M4 barrel was damn close to .750 at the rifle legth FSB spot, while I measured a DPMS M4 that I have and it was like .020 smaller at the same area. So if you see a barrel you think you like you need to call and have them give you the real actual measuremeant at that location. I think most shops won't pin a FSB if the barrel isn't .750. You can always use an Armalite clamp on FSB of even make a set screw FSB by just drilling / tapping a used A2 FSB. |
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