BCG pressure doesn't drop sharply after bullet exits muzzle ??

**StainlessSlide** · 11-18-12, 00:31

I have always wondered how long it took BCG expansion chamber pressure to rise after the bullet passed the gas port, and how long it took to fall after the bullet exited the muzzle. I didn't expect much latency at all.

This paper

Aberdeen paper - WMWerner

shows the BCG expansion chamber pressure (which they call the "pressure at the carrier cavity") rising over a period of about .6 millisecond, and falling over 2 ms (see page 29). This is quite a surprise, as we know that dwell time for an m16a1 (which is what they are using) is only about .2 ms.

The same kind of plot appears on page 23, but the pressure axis is off by a factor of ten. The paper is not very well written. They say that t=0 is the muzzle exit time (which they call the "shot ejection time"), but BCG pressure doesn't start to rise until *after* t=0.

Anyway, if these measurements are correct, a serious rethinking is in order.

By the way, the max BCG velocity of about 15 ft/sec given on page 22 is in close accordance with other measurements I have seen, and also with my own modeling.

**Gun** · 11-18-12, 10:56

Originally Posted by StainlessSlide

shows the BCG expansion chamber pressure (which they call the "pressure at the carrier cavity") rising over a period of about .6 millisecond, and falling over 2 ms (see page 29). This is quite a surprise, as we know that dwell time for an m16a1 (which is what they are using) is only about .2 ms.

The same kind of plot appears on page 23, but the pressure axis is off by a factor of ten. The paper is not very well written. They say that t=0 is the muzzle exit time (which they call the "shot ejection time"), but BCG pressure doesn't start to rise until *after* t=0.

If you look at the plot on page 23, the carrier starts rearward movement at ~ 0.2 ms after the bullet exits the muzzle. Is this not the (ideal) dwell time?

**markm** · 11-19-12, 10:06

Originally Posted by StainlessSlide

Anyway, if these measurements are correct, a serious rethinking is in order.

Why? What needs to be rethought?

**jstone** · 11-20-12, 00:11

Originally Posted by markm

Why? What needs to be rethought?

that is what Im wondering. What seriously needs rethinking.

**markm** · 11-20-12, 07:09

The whole design needs to be scrapped. It's just NOT WORKING!

**davidjinks** · 11-20-12, 07:26

I don't even know why this is a viable thread.

Notice the DATE:

I'm pretty sure a lot of stuff has changed since then.

**TMS951** · 11-20-12, 07:54

A lot has changed since then. However the operating system of an ar15 has not. Still I do not see the problem with how long it takes the pressure to drop.

**MistWolf** · 11-20-12, 08:47

Some think the bullet is still in the barrel when the action starts moving. Some also think that "dwell time", the time the bullet takes to travel from the gas port to the muzzle makes a difference in BCG velocity.

While the distance from port to muzzle does make a difference by increasing blow down time after the bullet exits, the time/distance the bullet travels from gas port to muzzle means very little, if anything at all