Canadian Airgun Forum

Curious request
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Author:  joe hickey [ Tue May 17, 2016 6:31 am ]
Post subject:  Curious request

I would like to ask any of my fellow modders who installed a " riser breech" on a pcp to do an experiment for me to satisfy my curiosity. I would like a " chrony " test done. five shot avg, with a riser breech, and, a five shot avg with the normal breech. A 2240 on C02 would also work. I am trying to determin if there is any loss in speed using the riser. And how much, if any. The same starting pressure, same pellet, and same barrel. My curiosity comes from the fact that the space from the valve seal to the back of the pellet is increased. And I was wondering how much speed is lost by that increase. For instance, if a riser breech were 10 inches hige, a substantial drop in speed would occur. But what happens when that increase in dead space, is only 1/4". Any help would be appreciated. Thank,s.

Author:  rsterne [ Tue May 17, 2016 9:07 am ]
Post subject:  Re: Curious request

I can't give you experimental results, but Lloyd's spreadsheet uses transfer port volume as a variable.... This is necessary because the pressure in the valve drops before the air gets to the pellet while it is expanding into the space between the valve seat and the base of the pellet.... This volume consists of the throat of the valve, the exhaust port, the transfer port, and the barrel (chamber) between the bolt face and the pellet (minus the loading probe, if any).... The smaller the volume of HPA available to expand into that space, the greater the pressure drop.... The worst case, in the Crosman guns, would be the 13XX pumpers, because they have the smallest internal valve volume, only about 1.5 cc.... A 22XX has about the same internal volume, but a bit of CO2 can bypass the piercing stem, which allows it to draw from the CO2 cartridge a little bit.... In a bulk fill, or PCP, much more of the reservoir volume can be included in the "valve volume", and in the case of a gun where the air can flow more freely into the valve than out, the entire reservoir is available to prevent pressure drop in the transfer port....

Therefore, Joe, the answer to your question will vary widely depending on the valve (reservoir) volume.... In a stock Crosman 13XX or 22XX, the volume between the valve seat and the pellet is about 0.3 cc in .177 cal and a bit less in .22 cal, because the bolt probe is larger.... In a 1377, with 1.5 cc of HPA expanding into 1.8 cc of volume, the pressure would drop 17% before the air gets to the pellet.... Increasing the height of the breech, and therefore the length of the transfer port, by 1/4", with a stock 0.140" TP, would all an additional 0.07 cc, making the total volume 1.87 cc, and the pressure drop 20%.... Therefore, the additional pressure drop caused by adding 1/4" to the length of the stock diameter transfer port would be about 3%.... That should case an equivalent drop in FPE, which means a velocity loss of about 1.5%.... The longer passage, which is smaller than boresize, could cause a slight additional loss, so my best guess would be a 2% drop in velocity....

As I said, that would be the worst case.... In a PCP with a large reservoir and large inlet to the valve such a small increase in TP volume might be unnoticeable.... However, if you are dealing with bore area porting, and long ports, there is no question that additional volume between the valve and pellet can cost performance....



Author:  Doc Sharptail [ Tue May 17, 2016 4:04 pm ]
Post subject:  Re: Curious request

Excellent question- one worth looking into. I'll see if I can get a regular XX SBK breech onto the 1701-P when I get back to work next week... right off the top of my head, I'll have to cheat with a bbl band at the very least.


Doc Sharptail

Author:  joe hickey [ Wed May 18, 2016 9:12 am ]
Post subject:  Re: Curious request

Thank,s Bob, that answers my question sufficiently.

Author:  Handyguy [ Sun May 22, 2016 11:11 pm ]
Post subject:  Re: Curious request

I installed the Archer repeater breech on my 2079. This raised the barrel about an 1/8" over the original. With the same valve, extended probe and just a slightly larger bore in the TP, I actually got higher fps over the original. Granted this is CO2, so maybe I'm just taking up space in this post.

Author:  rsterne [ Sun May 22, 2016 11:54 pm ]
Post subject:  Re: Curious request

The larger TP probably gained more than the tiny loss of making it 1/8" longer.... or the temperature may have been a few degrees warmer, that's all it take for a gain with CO2....


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