Rifle specs are
P415 14.5 pinned upper
ST lower
ST-T2 buffer/spring milsepc
ST lower kit
Magpul UBR stock milsepc

For some odd reason the buffer tube is getting some decent gouges right at the end of the tube where the buffer sits. This is less than 2,000, prolly sitting around 1,500 rounds of mostly all Lake City green tips and a few hundred tula/Amercian Eagle 55gr.
It had issues with FTE/FTF and alot of brass shavings in the bolt but I polished the bolt and it runs like a top.
She eats anything, only other problem currently is the bolt will not lock back to the rear 8/10 times no matter what mags, ammo I use. That I can deal with the bolt not locking back but the damage done to the tube with these few rounds has me a little worried.

The deepest marks are at the TOP of the tube, which has me confused because carrier tilt would leave marks on the bottom of the tube. The rear of the carrier is flat, flush and smooth as silk(i dont belive the carrier is hitting the tube as the finish that POF put on it is near perfect)

As of right now these are all the pictures I have of it, I can get more.






Anyone have anything on this? This is my SHTF gun and right now its a huge ? on long term reliability.

I dont believe so, the coating on all of the carrier is near perfect. I'll get more pictures of the carrier, tube and buffer tom.

The spacing between the marks on the tube are the same distance as the flat parts of the buffer, as you can see in the pic of the buffer the wear marks showing the alum. under the coating on their buffer. Im quite certain the buffer is hitting the tube, but not sure what is causing it to push upward? I did not notice any finish defects on the buffer, tube or spring when I installed them. I did how ever compare the ST-T2 buffer to an off brand(grey in color) buffer that is in my buddies AR and the length of the head of the buffer is about 3mm shorter on my ST-T2 buffer and does not have flat cuts on the head either. Thanks for the reply.

Dano could you please clarify what you meant by cutting the tube? Do you mean notch the bottom of the tube so when the tube is threaded in fully the retainer pin sits in the notch?

Im not sure but the UBR stock dosnt allow for indexing, you tighten the tube onto the lower (use wrench to snug it to lower) then slide on the stock and tighten the stock down with the supplied screw at the end of the tube.
The tube is snugged to the lower/wont go any further.

I just dont understand why its doing it to the top of the tube tho. These marks are DEEP gouges, they are measurable. Pics later in the day today will show this, ill take it all apart and get some good detailed pics of it.

If I misunderstood anything from your reply I appologize. Stay tuned for more pictures!

Buffer spring is not the item that keeps the key from crashing into the back of the receiver, it's the length of the buffer.

A longer than needed spring is just going to coil bind up before before the carrier/buffer has fully retracted before the buffer bumper makes contact with the back of the tube.

So, with the correct buffer in play, the face of the bolt should stop about 1/4"to 3/8" in front of the back of the ejection port rear edge.


And as for what is making the marks in the tube, is the the larger section just before the reduction steps in the back of the carrier( look just behind the 9/10 marking on the carrier OD,and you can see the wear areas on the carrier that is casing the damage since they are worn/marred as well)


So the question comes in play, what is off between the upper and lower receivers with tube installed that is causing the carrier to strike the tube off center?