Alright so i purchased a Taccom upper a few months ago. Until today i have only used one lower. The lower is a FMK lower, psa MOE lower parts kit. and a spikes pistol plug. at first I thought it was the upper giving me fits; however, I put the upper on a lower with a buffer tube, buffer, and buffer spring. the upper shot perfect. When I use the upper on the pistol lower i only get about one or two rounds to fire. I modified the spikes pistol plug with a taccom pressure plug to add pressure to the BCG, that increased the shots fired to about three or four per mag. So can anyone think of why a buffer and spring would make an upper fire more reliably than one without?
Thanks in advance for the help. it is appreciated

What type of issue are you having? Feed, extract, eject, etc.?

Every second or third shot I will hear a click and no boom. So light strikes. But when I put the upper on a regular ar pistol lower it works perfect. The upper won't work with a spikes pistol plug.

You are sure the bolt is all the way closed when you have the failure to ignite?

What do the rims look like... 1) on the fired cases, 2) on the ones that don't fire?

The ones that do ignite have a well defined dent from the firing pin, the ones that do not ignite have just a slight mark. I am sure that it is in full battery when it the light strikes occur.

Are you running the same fire control group in both lowers.  Some hammers do not have enough clearance and stike the underside corner of the bolt.  This can keep the rifle from firing and leaving slight to no firing pin on strike showing.   Some notched hammers are bad about this and generally not recommended for rimfire uppers.  Broken firing pins or broken firing pin springs can produce erratic ignition.

I've used my colts hammer which is notched. Because I thought that was the problem when I first got the upper. So I'll switch it back. And see if that helps

Try a "dry build" and look under the assembled upper.

(I assume you have thoroughly cleaned and inspected your bolt and chamber area. That should be done up front. Check for any signs that the bolt parts and chamber area don't have issues first.)

In other words, assemble the upper and place the bolt in the closed position. See where the back of the pin and the back of the bolt line up. Does the back of the firing pin look right?

If you have the instruments, you can measure from the main mounting holes to the back of the closed bolt to see if the hammer needs to be shaped for a clean strike on the pin, but I realize not everyone has inspection tools.

If the hammer appears to be falling wrong, after enough cycles there will be evidence of abnormal contact with the bolt. Normal cocking action and firing strikes don't make the same marks as abnormal ones, but it takes many cycles to become obvious. It is easier to debug by trial than to run it bad long enough to see the marks.


Those rim observations are a key. Now we just got to back into narrowing those causes down and she should run like a top.