I have a Tac Sol M4 .22 upper I just got used but in new condition, doesn't even appear fired! Put it on my Anderson lower with RRA varmint 3.5lb trigger and it fires one round only!!!!, No matter what ammo, mags, leaving buffer/spring in, etc. Next rd in mag is chambered but no trigger re-set. Also sometimes in chambers the next rd, FTE 1st rd fired, and still no trigger reset. It smashed one empty case forward so hard it chipped the top corner of the collar. So is it the lower or the trigger with this upper only. Don't know what would cause this??????????????????

Now here is the kicker #1, this is the same lower my Tac Sol SBX barrel is on and it runs flawlessly!!!!!!!!!!!, no matter what mags or ammo I use, no malfunctions at all and super accurate.

Kicker #2-I put said M4 upper on my Spikes lower and it ran 2 mags almost flawlessly, 1 FTF and 1 FTE most likely rem olden crap bullets.

I measured but found my problem...............the spikes lower has a mil-spec trigger with jp ent. red springs, very easy to reset but heavier 2 stage pull. The Anderson lower in question with the RRA varmint 3.5lb pull is harder to reset and has 2 reset contact points but easy 3-3.5lb pull. I pulled both Tac Sol bolts  and they are NOT identical. the backend on my newer SBX barrel in .993 in, the M4 bolt is .990in, I tried the M$ bolt in my SBX upper and did the same thing 1 shot. It also has a square angled bottom the assists in resetting the hammer and it is shorter than the SBX BCG same area not as much meat. I will call Tac Sol to see if the M$ is an older design or out of specs, instead of having to use it in one of my other lowers only..Yee Ha......

Yep, Thanks again. I need to set this up right bc this upper/lower combo will be for fun with a slide fire stock coming today since they slashed the price on them due to 2015 shot show new model coming out soon. The dealer on-line and around still pricing them at $370.00 Avg but Slide Fire's website has them for $199 now. Tac-con 3mr trigger is out to expensive and on multiple reviews & videos no one can get it to fire as fast as the marketing video or as continuous except the guy who ran it with a slide fire stock now that was FA fast but to expensive IMO.

I have a Tac Sol M4 and an early LT.I also encountered problem's.And,I learned that Tac Sol's will not function properly with a 2 stage trigger.I put them on a lower with s single stage trigger and,I haven't had a problem since.
By the way, an average Human Hair is .002"-.003" thick.So,I doubt it is your Bolt.Try a single stage trigger and see what happen's.

They do work with 2 stage triggers, both of mine due, just the older M4 bolt wont work with the RRA varmint trigger (harder to reset) and the bottom of the bolt surface area is smaller than the new bolt. I called Tac Sol yesterday, they re-designed the older bolts from the M4 & LT.1, my SBX has the newer bolt it measures .993 at the back of the BCG, the  M4 is .990, the newer bolt wont fit in the M4 receiver it fits all the way until the back of the BCG that's .993 Tac Sol advised that shouldn't be and was going to just send me a new bolt until I explained that, they advised me to send the whole upper(M4), they would fix or replace anything I needed. Great customer service!.  BTW my older M4 worked perfect on another lower with a mil-spec 2 stage trigger. (much easier to reset than the RRA varmint).

I have a Velocity single stage coming tomorrow from Molon Labe it's 3-3.5lbs and all video's & reviews said it was great, better than CMC, etc That trigger plus my slide fire stock and M$ upper when it comes back should be a fun little combo.