Does the buffer typically bottom out on the back of the buffer tube?  

What is the minimum travel for the BCG to cycle correctly?  Breach face to bolt face...  


Not sure why the picture isn't showing up, I can see it in preview but after submitting it only shows as a link?

The buffer is intended as a fail-safe, to prevent the gas key on the carrier from impacting the buffer ring on the lower receiver. If the gas system, buffer, spring and cartridge pressure are all in harmony, ideally, the buffer will never bottom out in the buffer tube (invoking its fail-safe function).

I suspected that..   Do you know if their is an optimal travel distance of the BCG during a cycle, mil spec. ?  

I'm trying to set the travel limitations of the BCG on the model.

Yes.



Measured on a functional upper receiver just now - 0.35" bolt carrier movement for bolt to unlock, plus 1.750" case length = 2.15" to eject a case; to pick up a new round, 2.75"; for last round lock-back, 3.2". HTH.

FWIW, that appears to be a semi-auto fire control group, but the selector is in "Auto" position; SA selector has no Auto position. Also, an unfired round is never retained on the bolt face, the buffer face is always in contact with the bolt carrier rear face, and there's no action spring shown. Very nice and artistic rendering, overall, though.

Ideal conditions with a tuned gas system, this might be true.
"Milspec" or gassed for reliability in all conditions, it will bottom out every time.

If it's running right the buffer does bottom out in the receiver extension like it's supposed to.

The AR15 design is not a low restitution system like the Ultimax. It's more of high restitution system.
Restitution refers to the energy of the reciprocating parts being returned to those parts. For lack of a better analogy think of restitution as a weird form of ricochet.  Smack two hammers together for a practical example of high restitution. Watch your kid bounce on a pogo stick for an example of low restitution.

Buffering is the secondary function of the buffer. Spring guidance is the primary function. Bolt carrier anti bounce is it's other function.