I am in the middle of assembling an upper for an LR-308. The upper is from a SI Defense receiver set, DPMS forward assist assembly, DPMS 308 forward assist spring, and DPMS BCG. I installed the forward assist into the upper. It's worth noting that this upper does not use a roll pin, it uses a an allen head srew supplied by SI defense. I put the charging handle and the BCG group in and the carrier will not come out because it is contacting the forward assist on it's way back. Am I missing something here?
-
Having trouble using the site?
Contact support
LR-308 Build Question: Forward Assist
- Thread starter daryar79
- Start date
Nothing like posting a question and then an hour later declaring you resolved the "problem" with a bench grinder!! 
Aside from getting an out-of-spec FA, installing it improperly to begin with, having the pin/screw hole in the wrong place on the upper receiver...not sure what to tell you. I have installed several FAs in SI Defense .308 uppers, including one DPMS FA just a couple weeks ago for a fellow Hide member and have never experienced any hang-ups with the carrier like what you described.
What did you determine the specific problem to be before flipping the switch on the bench grinder and going to town with it?
Aside from getting an out-of-spec FA, installing it improperly to begin with, having the pin/screw hole in the wrong place on the upper receiver...not sure what to tell you. I have installed several FAs in SI Defense .308 uppers, including one DPMS FA just a couple weeks ago for a fellow Hide member and have never experienced any hang-ups with the carrier like what you described.
What did you determine the specific problem to be before flipping the switch on the bench grinder and going to town with it?
I'm sorry, I was long on aggravation and short on patience. I am certain that I installed it properly. That's what had me stumped. I know the FA was correct to the build, I got the 308 spring, and it was installed properly. I also have a lot of faith in SI Defense. So when those possibilities are eliminated, what are you left with? For me, it's the bench grinder....lol. I mean, I've never really had a need for a FA anyway. FTF's and FTE's are never resolved by that little clump of engineering garbage anyway. Ask Vietnam Vets. Either way, I didn't take off a lot, it still functions the way they all do...and my BCG isn't hung up. So this time, ye ole grinder worked, but you're right, sooner or later it'll cost me.
Did you install the pawl 180 degrees off by any chance, or depressed passed the point of roll pin captivation?
The bench grinder is for grinding cut-down FSB's. I have used it to taper the ends of roll pins, but there are better methods, like chucking the roll pin in a drill.
The bench grinder is for grinding cut-down FSB's. I have used it to taper the ends of roll pins, but there are better methods, like chucking the roll pin in a drill.
79:
you aren't missing anything. S.I. has chosen an incorrect location to place the assist retaining screw. the hole should have been placed on the diameter of the forward assist hole. they chose to put it .015 past the diameter. additionally some uppers were made with the hole a full .020 from the correct location. because of this, when the assist retracts the "pin" portion of the screw won't contact the small bump on the pawl and depress it to push it back from the carrier.
S.I. is adamant that the location is correct and their "fix" is to grind off the offending portion of the pawl leaving only a small area for it to catch the carrier.
i had a few of their sets that were made in march of last year. that were out of tolerance closer to the correct location and they functioned correctly, however later in the year and since then they have settled on that defective .015 off location dimension and the assists just don't work right. i have just spent about 10 hours working out all the dimensions i needed, building 2 fixturing details and grinding 2 special cutters to relocate the screw hole to the correct position. my forward assist is now perfect, no thanks to the folks at S.I.
you aren't missing anything. S.I. has chosen an incorrect location to place the assist retaining screw. the hole should have been placed on the diameter of the forward assist hole. they chose to put it .015 past the diameter. additionally some uppers were made with the hole a full .020 from the correct location. because of this, when the assist retracts the "pin" portion of the screw won't contact the small bump on the pawl and depress it to push it back from the carrier.
S.I. is adamant that the location is correct and their "fix" is to grind off the offending portion of the pawl leaving only a small area for it to catch the carrier.
i had a few of their sets that were made in march of last year. that were out of tolerance closer to the correct location and they functioned correctly, however later in the year and since then they have settled on that defective .015 off location dimension and the assists just don't work right. i have just spent about 10 hours working out all the dimensions i needed, building 2 fixturing details and grinding 2 special cutters to relocate the screw hole to the correct position. my forward assist is now perfect, no thanks to the folks at S.I.
Last edited:
Which is why my first build was with and Armalite slick sided upper (no Fa). I suppose if one were to build a FullAuto and got into a zombie spray fest shootin a bunch of military tar sealed ammo, then a FA MIGHT come in handy. might.
Similar threads
