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bf1956
(@bf1956)
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.177 TP size differ from a say .20 to .22, .25....?


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Jim Bentley
(@jim-bentley)
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Not that I have ever seen. 


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Limbshaker
(@limbshaker)
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No. It's around 3mm on all of them. 


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straitflite
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Posted by: @limbshaker

No. It's around 3mm on all of them. 

IIRC, I’ve never heard of a springer manufacturer sizing a given make/model’s TP differently for caliber. Does anyone know if that is a correct statement or of any exceptions?

Maybe same power plant needs same resistance at that point of the shot cycle regardless of caliber?

Makes me scratch my head a little 🤔 since pellet weight generally goes up with larger caliber. Curious thing I suppose. Cardew write anything on the subject or is it simply a matter of economics?   Seems like there should be mathematical science to this stuff. 🤔 


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Limbshaker
(@limbshaker)
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The diameter of the hole isn't all of the equation. The length of the port has a huge effect on how it performs. 

It's also dependent on how the powerplant is designed to operate.

For instance, leather sealed guns typically have bigger transfer ports and have more "slam" at the end of the stroke. The leather isn't as efficient in sealing, and builds less peak pressure. 

The older Weihrauchs with leather seals generally had 4mm ports, while all of their current offerings that I know of use the 2.9mm port. 

Piston bounce is where accuracy gets sacrificed, and some target guns had minimizing this in mind with their design. Here are two that I have:

The FWB300 was designed with a huge, short port but had a very efficient sealing piston ring. This would typically make for a lot of slam, but the limited volume and generally tight pellet fit at the bore offset the large port. But the fact that the piston has a rubber bumper in its face shows that they intended for it to land hard and have the bare minimum of bounce to "cushion" the pistons landing.  

The HW55 with leather seal used a very light piston and light spring, which would normally cause a ton of bounce, again robbing accuracy. They used a 4mm port to reduce peak pressure and prevent it. 

Keep in mind that a low peak pressure sacrifices power output. You have to find the right tradeoff of minimal piston bounce but without slam, but still make decent power. Pellet fit and seal design also have huge effects on this in addition to transfer port volume. 

It does seem odd that Weihrauch has decided that the 2.9mm port is the best compromise for all powerplants. Are they right? Well I don't know. But I do modify the ports on my 77/97s and get good results. 

Interesting reading for the nerds: 

http://www.airgunbbs.com/showthread.php?690110-Stroke-lengths-TP-sizes-and-SCR-for-various-springers


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bf1956
(@bf1956)
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Topic starter  

Oh and if I may, when did the R9 come with a delrin guide? I've only done a R8 and thought that guide was really crude. I don't take actions out of there stock if I don't have to. The R8 was a steel tube with a four petal base. Oh and one other question, how or what do I use to measure the TP ?


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Limbshaker
(@limbshaker)
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I can't confirm with certainty, but I believe the R9 always had a plastic guide with a steel base washer. I've never seen one with the old steel guide. 

For the transfer port, I measure them with gauge pins just because I already have them. 

One good easy thing to use is a wire gauge drill bit set. They are so handy that you should have one anyway. The drill bits go up in increments of a few thousandths at a time. Just slide the smooth end into the hole and then measure the one that fits the best with calipers. 

Harbor Freight has them. 

https://www.harborfreight.com/titanium-high-speed-steel-numbered-drill-bit-set-60-pc-61690.html

As for the length of the port, you'll have to stick something through the port, mark it, pull it out and measure. Not real precise that way, but that's about all you can do easily. But I think that's really close enough.


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straitflite
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Posted by: @limbshaker

The diameter of the hole isn't all of the equation. The length of the port has a huge effect on how it performs. 

It's also dependent on how the powerplant is designed to operate.

Thanks for the info. I hadn't thought about TP length. Clearly you are more enlightened than I so your explanation helps with more understanding.

Switching to nerd mode 🤓, Bernoulli's principle of convergent & divergent designs is what affects velocity and pressure. With that, a chamfered TP on the breech side would be divergent by design and thus more pressure at the pellet. If one was to chamfer which chamfer angle would work best for the application with regards to the chosen gun/caliber....60 degrees? 40 degrees? If TP length and diameter are taken into account I "feel" that could be calculated. I'm just thinking out loud here so I'm not expecting an answer really. Just pondering on the forum LOL I don't have any maint tubes lying around that I can afford to practice with 😱 

The P1 for example has a TP with a 90 degree bend so on a recent build with that pistol, I didn't chamfer but I did hone out both ends on the order of some odd thousands so the ends are now bored out slightly larger. At the 90 degree point, it remains the same diameter because I obviously cannot reach that bend with a straight tool. So the compressed air reaches the first bored out section from the main tube, with more area to transfer the pressure then travels through the untouched smaller (convergent) 90 degree bend causing the velocity of the air to increase and pressure to drop, then it travels through the bored out section at the breech causing pressure to rise again at the pellet, -following Bernoulli's principle. (did I mention I was switching to nerd mode?) I'm getting 6.5fpe from my .177 P1 which is pretty darn good but not completely run in yet and has the Australian parachute seal, ....so long term results remains to be seen -or understood for that matter.

Might be a crazy post but I can certainly see the great importance of TP design. Thank you for the link btw.

Posted by: @limbshaker

I do modify the ports on my 77/97s and get good results. 

What kind of mods did you do on your 77/97 if I may ask?


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Jim Bentley
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I believe that “if” a chamfer was to be used to an advantage it would be on the compression side. 


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bf1956
(@bf1956)
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That was my initial thought, I am here to learn. What brought this OP on was the recent return of a R9 .20 chopped, choked and crowned barrel. Upon reassembly and a couple hundred rounds of different pellets I get a remarkable group (1/2 in.). Then just like that it goes the complete opposite, don't matter the pellet. JSB,Benjamin, Webley and one other all performing stunning results then crap. I think I will change chamber seal see if that makes a diff. All connections good, even double checked. Prior to the mod I was shooting with a peep at spinners cans and what not. So really don,t recall prior accuracy.


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straitflite
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Posted by: @jim-bentley

I believe that “if” a chamfer was to be used to an advantage it would be on the compression side.

That would result in higher air velocity at the pellet with drop in pressure. You may be right though, physics can be pretty wild. A choked barrel is also convergent by design but usually with a pellet in it...my head hurts LOL

bf1956, please accept my apologies if I have hijacked your thread. TP's baffle me somewhat LOL

Of course the usual response for sudden loss of accuracy is to clean the barrel as you no doubt already know. I hope the seal change will be the fix for you. When you say "chamber seal" did you mean breech seal?


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bf1956
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@straitflite  You're all good, no apologies and yes the breach seal. Yet I missed the point of of my Op, I thought about changing out thr barrel to a .177 to check accuracy. Now this is when I thought maybe the TP,s are sized according to cal. Seems to me I did a visual Awhile back when waiting for the .20 and it appeared larger than a.177. I am going to get the wire bits as Limbshaker recommended and then I will check out various TP,s. Oh and that is the otherthought is cleaning the barrel for it has had a bluing at the new crown.


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Limbshaker
(@limbshaker)
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IMG 20210516 175431258

First thing I do with a new or modified barrel, is push a pellet through and feel it out. You can do it without removing the barrel. 

Take a pellet that seems to shoot well, put it in the breech, and push it through slowly through the muzzle. If the muzzle end is more loose than the rest, toss it. Probably will never shoot well.

Also pay particular attention to the crown, right before the pellet "pops" out. If the crown was done right, the pellet won't hang right before popping out. If it hangs or takes more pressure to "pop", the crown needs work. I've done a few crowns in the lathe before that looked excellent, but had this issue and shot terrible. 

I like to check the barrel feel first, before wasting time checking other things. That's just me, and how I like to do it.

Other than that, clean the barrel well, and shoot at least 25-30 pellets to see if it gets better. Sometimes they need a little leading up to come into their own. 

 

 


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bf1956
(@bf1956)
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@limbshaker  Did the pellet push through and felt a nice choke, and i will repeat and feel more for the exit. Will also do a good cleaning before hand. Thanks...


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Jim in UK
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Posted by: @limbshaker

 

Take a pellet that seems to shoot well, put it in the breech, and push it through slowly through the muzzle. If the muzzle end is more loose than the rest, toss it. Probably will never shoot well.

 

Before scrapping a barrel with a slack muzzle, it might pay to try the following, for which you need some water, an empty margarine or similar plastic tub, a deep freeze and a blowtorch.

 

Use the water, tub and freezer to make a block of ice. Then, heat the last 1" of muzzle to a dull red, which causes the metal to expand and makes it malleable, then roll it on the ice, which shrinks the outside, and squeezes the bore. Rod a pellet through, and see whether you've now got a choke and, if not, repeat until you do.

 

Got to be worth a try,


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Limbshaker
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Posted by: @jim-in-uk
Posted by: @limbshaker

 

Take a pellet that seems to shoot well, put it in the breech, and push it through slowly through the muzzle. If the muzzle end is more loose than the rest, toss it. Probably will never shoot well.

 

Before scrapping a barrel with a slack muzzle, it might pay to try the following, for which you need some water, an empty margarine or similar plastic tub, a deep freeze and a blowtorch.

 

Use the water, tub and freezer to make a block of ice. Then, heat the last 1" of muzzle to a dull red, which causes the metal to expand and makes it malleable, then roll it on the ice, which shrinks the outside, and squeezes the bore. Rod a pellet through, and see whether you've now got a choke and, if not, repeat until you do.

 

Got to be worth a try,

Have you ever actually tried that? I have more than once and couldn't get it to work, and the bore just ended up with nasty rough scale on the inside. 

 

 


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Jim in UK
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Posted by: @limbshaker
Posted by: @jim-in-uk
Posted by: @limbshaker

 

Take a pellet that seems to shoot well, put it in the breech, and push it through slowly through the muzzle. If the muzzle end is more loose than the rest, toss it. Probably will never shoot well.

 

Before scrapping a barrel with a slack muzzle, it might pay to try the following, for which you need some water, an empty margarine or similar plastic tub, a deep freeze and a blowtorch.

 

Use the water, tub and freezer to make a block of ice. Then, heat the last 1" of muzzle to a dull red, which causes the metal to expand and makes it malleable, then roll it on the ice, which shrinks the outside, and squeezes the bore. Rod a pellet through, and see whether you've now got a choke and, if not, repeat until you do.

 

Got to be worth a try,

Have you ever actually tried that? I have more than once and couldn't get it to work, and the bore just ended up with nasty rough scale on the inside. 

 

 

Yes I have, and it worked insofar as it created a choke in a length of scrap barrel after four heat/freeze cycles.  

 

What I have not done is try it on a barrel to test before and after accuracy, because I have not had a barrel with a slack muzzle since.

 

Like I said, it's worth a try before junking a barrel.


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bf1956
(@bf1956)
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Topic starter  

Trying to salvage a Big Box store .22 with more choke at the chamber and non at the muzzle made me think the barrel was installed backwards. I tried a 1in. pipe cutter with the cutter wheel removed and did get some choke, need to get a larger pipe cutter. Just some musings to throw out there.


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hkshooter
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I had a tight grouping R1 take a poop one day. Shot fine earlier in the day, in the afternoon couldn't hit a hare at 20 yards. Did some testing, the rifle didn't group anymore, it patterned. I sent it to Beeman for a look over, the spring was toast. They replaced the spring and refreshed the breech seal and the gun was tight shooting once again. It had zero to do with the choke or tp, which was as the factory left it.


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bf1956
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Cleaned the barrel and doing better, best group 20yds. dime size with Benjamin pellets. I would think the JSBs or Webley's would perform better but not the case. I do have a ARH kit in house if I decide to go that Direction. Thanks to all for the input.


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