Small Chamber 13xx ACP, Questions.

[DavidS]

I am doing some research with the intention of putting togather a good 1322/2289 platform ACP rifle.

Has any one yet made an ACP based on the 13xx platform that uses only the standard valve bored out, and cut to maximum possible safe volume and nothing more for the air chamber?

Also is it better to leave the stock valve seat for the exaust popet valve, or is it better to make a 60 degree recessed seat for consistancy?

Now obviously there would be all the standard mods to the piston, vavle nose, transfer port, and the springs would be tuned for the ACP aplication (with a power adjuster on the hammer spring).

I have also have thought that making a breach to use the M-Rod clips would not be to hard, has any one made such a breach yet?

[rsterne]

The largest I have managed to bore out a 13XX valve is to 2.2 cc, still not even close to barrel volume, let alone larger.... Using that, I did manage to achieve two shots of 600 fps with a 14.3 gr. pellet in my Uber-Carbine.... To get more shots (or more powerful shots), you need a much larger valve.... and a pressure indicator so that you know you have arrived at the proper fill pressure....

BNM makes a breech to use the MRod clips on a 22XX platform, no reason it couldn't be used on a 13XX.... It would have the same fitment issue as the Crosman steel breech as they are made for the larger 22XX tube diameter.... They are well over $100, however.... I would question why an 8-10 shot breech is necessary for a 2 shot gun....

Bob

[DavidS]

Thank you for that.  2 shots at anything over 540FPS would be great so long as the spread is not to great.

And for a pressure indicator I intend to use a spring loaded screw pop out pin like some use with the Benjiman 39x ACP's.

[DavidS]

The largest I have managed to bore out a 13XX valve is to 2.2 cc, still not even close to barrel volume, let alone larger.... Using that, I did manage to achieve two shots of 600 fps with a 14.3 gr. pellet in my Uber-Carbine.... To get more shots (or more powerful shots), you need a much larger valve.... and a pressure indicator so that you know you have arrived at the proper fill pressure....

2.2cc?   Taht does seem large, may I ask: how did you get that much volume?

I know about cutting the threads back a bit, and drilling into the back around the transfer port (making sure not to get to close to the transfer port itself), and hogging out the ID a small amount.   What else could be done with the stock 13xx valve to get greater volume?

BNM makes a breech to use the MRod clips on a 22XX platform, no reason it couldn't be used on a 13XX.... It would have the same fitment issue as the Crosman steel breech as they are made for the larger 22XX tube diameter.... They are well over $100, however.... I would question why an 8-10 shot breech is necessary for a 2 shot gun....

Bob

If I were to use an MRod clip I would make the breach for it myself.   As the MRod clip is self feeding and relies only on the bolt being extracted and inserted to rotate (the missing pellets alowing it to rotate) I think it would be quite simple to do.

As to using a 8 to 10 shot breach on a 2 shot gun, well it is just a convinience for loading those two shots .

Also do you mind if I ask:
How much is left in your valve on the ACP that you mentioned here after the second shot?  And how many pumps does it take to get back to full pressure after 2 shots, and after just one shot?

Again thank you very very much for this information.

[rsterne]

The threads were shortened to 0.30", and the rear portion bored out to 0.500" for a length of 0.400" from the back (valve seat).... The two shots completely empty the gun.... I used a combination of balancing the Disco hammer spring preload and the number of pumps to achieve equal velocity on the two shots.... You can find several combinations that will work, each producing a different velocity for the two shots, but requiring a different number of pumps.... I was using the equivalent of 30 pumps to get the two 600 fps shots.... My Uber-Carbine uses a 2200 pump tube and linkage with 50% greater swept volume, so it only requires 19 pumps instead of 30.... Here is an example of two settings.... Where the first and second shot velocities meet is the tuning point.... The second tuning point (Disco spring maxed out) would require nearly 50 pumps with a stock 13XX/XX89 pump with FTP....



I presume you realize that the MRod clip requires a MUCH deeper breech, trying to fit one into a Crosman steel breech will result in cutting it in half completely.... You will have to start from scratch to build a repeater breech.... I can tell you from experience it is not an easy task, you will soon decide that $100 is cheap!.... I would suggest you think about a 2-shot shuttle where you load 2 pellets and slide the shuttle side to side to line them up for loading.... You MAY be able to cut away a Crosman breech to accomplish that, I don't know.... To get enough rigidity, you will need an additional mounting point at the front of the breech....This could be done with a barrel band immediately in front of the breech, however....

Bob

[DavidS]

Thank you for the aditional info.   I will take into account.

As to the breech:
I am accustomed to making my own parts from metal stock.    Though I could see possibly having to have to make to high of a brech to avomodate the MRod clip.  So I may make a shuttle instead.

[rsterne]

When you finish your breech, make sure to post a photo of it.... seeing what others do is always interesting and informative....

Bob

[silent_airman]

What does ACP stand for? (45 ACP pops into my head, but I know it is not that)

[rsterne]

"Air Conserving Pumper".... They are also known as a "Retained Air Pumper" or RAP....

Bob

[silent_airman]

"Air Conserving Pumper".... They are also known as a "Retained Air Pumper" or RAP....

Bob

Hmm, is that like putting a brick in the tank of your toilet to use less water?

[rsterne]

Kinda.... you close the valve before the pellet exits and flushes the remaining air....

Bob

[silent_airman]

Kinda.... you close the valve before the pellet exits and flushes the remaining air....

Bob

I take it that remaining air is wasted, since it is flushed after the pellet is downrange. That is air that took pumping to fill the valve, hence wasted effort. Even a small amount adds up after awhile. Am I close?

[rsterne]

Here is a recent thread that should answer most of your questions....  http://www.gatewaytoairguns.org/GTA/index.php?topic=69264.0

Bob