Air Gun Home :: View topic - 1377 Reducing transfer passage volume for increased velocity

Robw · Veteran Member

Today while shooting the 1377 it blew a transfer passage gasket.

I installed a piece of high performance fuel line as a make shift repair
so we could continue shooting.

I expected the velocity to drop as the tube ID is 3.8 mm and the transfer
passage is 6.3 mm. That is a reduction in diameter of 2.5 mm

It raised it's average velocity to 813 fps. It also made the pistol a lot
louder. I can only assume that the reduction in transfer passage volume
made it so that it had to compress less air into the large passage which
resulted in a greater volume and higher pressure pulse of air moving
through the machine.

Any other ideas as I would like to pursue this line of improving flow
and pulse strength?

Rob

rsterne · Moderator

The original "gasket" was far from stock (looks like a piece of inner tube with a hole punched in it), and in addition to having too much volume (as you correct surmised) you were probably losing air between the valve and tube, and also the breech and barrel.... Your "fix" puts the gun back to the way it should have been from the factory, with the valve sealed directly to the barrel (or at least to the breech)....

Bob

Robw · Veteran Member

There was NO air leak at all with the gasket until it blew. It was completely airtight.

I am going to try a few different IDs to see if I can find it's sweet spot.

rsterne · Moderator

How did the gasket seal the valve to the inside of the tube, or the breech to the barrel?.... It would appear that the original gasket was between the tube and the breech, both of which have a 1/4" hole in them.... I'm sure it sealed that joint just fine.... However, it would be nearly impossible to detect air leaking out between the valve and the inside of the main tube passing back into the hammer area.... or between the breech and the barrel passing forward (and back) along the sides of the barrel.... Did you determine that was not the case, and if so, how?....

Your new transfer tube sits against the valve directly (passing through the hole in the main tube), and against the barrel directly (passing through the hole in the breech).... It therefore seals the valve directly to the barrel, preventing any leaks that may have occurred as I described above....

Bob

Robw · Veteran Member

No, your first paragraph is entirely wrong.

How can you seal the breech to the barrel when
it is part of the barrel?

But, your second one is spot on except for the
tube seals the compression chamber exhaust port
directly to the inlet hole in the breech. It passes
through the receiver without there being ANY need
for it to be sealed to said receiver. Then, The bolt
pushes a pellet into the breech through the forcing
cone and the o-ring on the bolt seals against the
opening of the forcing cone.

A lot of people mistakenly refer to the forcing cone
as the chamber, which it is not. There is no chamber
in an airgun as the projectile is "chambered" into
the rifling in the bore of the barrel at the breech end of
the barrel and awaits discharge from there.
A firearm that takes a cartridge has a chamber machined
in to the breech which is housed in the action, usually the
same shape as the cartridge, that the cartridge sits
in until fired. Most chambers have a forcing cone
that guides the cartridge in to the chamber.
The chamber contains and bears most of the
pressure generated during firing.

The last part is from my Dad, a retired gunsmith.

Rob.

rsterne · Moderator

Robw · Veteran Member

Whoa! That's quite the attack on the new guy you are launching!

What are you, the welcome wagon or the forum drunk?

Well, I had fun experimenting with it anyway and I learned a few things
along the way.

I hope that you have a nice day.

Rob.