CZ200 tuning question

[cmlynch]

I'm tuning a CZ200T. I've drilled out the barrel transfer port to around 3mm. My receiver had no adjustment screw. I've swapped out the stock hammer spring with a much stiffer one. I'm currently averaging over 17 fpe with various weight pellets. I'm pretty happy with the power even though I have adjustment left in the hammer. In fact I don't want to give it any more hammer because I'm at the point where the curve starts at a higher pressure than I'm running. 200 bar tube and the curve starts up over 210 with JSB Monsters. If I pressurize to 200 bar I still get 42 shots inside 30 fps and a really good 26 shots inside 1% or 8 fps. I'm very happy with those numbers but I would love to add a few more shots without having to exceed recommended max pressure or losing the consistency that I've got.   

Here is my question: What effect does enlarging the transfer port typically have on the location or shape of the shot string? I wanted to ask because I don't get a do-over if I go too far drilling it out.   

Any other tips welcome obviously! Thanks

[cmlynch]

bump

[Ribbonstone]

Hate it when a post just hang-fires. Been playing with a pair of them (.177/22) for a few years, so I'll go ahead and put in my 2 cents.

I’ve not had to drill one, but 3mm  (call it .12”) seems like its large enough to take care of air flow to a .177 barrel.

Quick answer to the TP question: Just have to assume that drilling one larger does the same thing as opening the transfer port restriction screw.  Generally you’ll use more air, get a bit more speed (see below), and decreased shot count.

How much shorter a shot count?  Darned if I know, it often gets disproportionally less once you reach a certain point (and the transfer point is not the only flow restriction between tube and pellet).

Maybe you know these rifles better than I do and are aware that there is a mechanical device in place that acts as a kind of flow limiter (parts diagram #33).  Listed as a “pott”, it’s basically a pre-chamber attached to the valve. Idea is that the air has to fill this chamber before reaching the valve, which acts as an air flow limiter.

(This actually makes sense for a rifle designed to be a 6- 12 foot pound rifle and get a lot of shots out of 122cc’s of air. So long as you are using a good bit LESS air than the volume of air in the “pott” per shot, it does seem to increase efficiency.)

I have not removed that part.  17 foot pounds in .177 is about as far as I could go (although I do keep mine running at 12 foot pounds no more than 180BAR).. There have been folks who have removed the “pott” in order to boost power, and it does seem to increase air flow and boost power, but they are often disappointed in the resulting shot count.
 

[cmlynch]

I've read about the pott but mainly in reference to really squeaking the last bit of power out of this tiny gun. I'm happy where I am powerwise at the moment. My biggest concern is that my acceptable shot curve starts at too high a pressure with my current hammer setting. VERY high with JSB heavies and HN Baras but still too high with Monsters. It's still bell shaped if I start at the recommended max of 200 bar but I know I'm losing shots on the front end. The shot to shot consistency is really outstanding right now.

I imagine the opening and closing of a transfer port has pretty universal effects on a shot curve. Would opening it move the shot curve to the right or to the left? Or would it not move it at all? I know that opening it tends to give more power but doesn't it also do things to the steepness of the curve and the consistency from shot to shot?

Like I said I can't experiment with throttling my transfer port. I have to drill it and can't go back once done.  Opinions appreciated

[cmlynch]

Making liberal use of the search function I think I found what I need here: http://www.gatewaytoairguns.org/GTA/index.php?topic=24299.20

Bunch smarty pants folks here. 

[cmlynch]

It started out very accurate for short distances. It was only putting out about 5.5 fpe. I've since ramped it up and routinely get 0.25 inch groups with HN Baracudas at 35 yards under perfect conditions which is as far as I have available to shoot. Other pellets hover around the .25 inch mark at that distance but don't quite match the Baracudas. 

[scrane]

The (regulated) Steyr LG100/110 has replaceable transfer ports that vary  in size from 2mm to 4mm. Steyr uses the smaller ports for their 7.5 joule 10 meter guns (2mm) and their 12 fpe British market guns (3mm). For higher power guns they use a 4mm transfer port. When using the 3mm port for 12 fpe the shot count is substantially less than the same gun with a 4mm transfer port  installed and the hammer spring tuned for 12 fpe. In other words, the 3mm port is less efficient and noisier than 4mm port at the same energy. The 2mm port is virtually useless outside of very low power requirements. This might not apply to your situation, depending on the position of the transfer port in the air flow, air pressure, and overall flow restrictions in the system. Do a search for "Steyr transfer port" for lots of information. 

[Ribbonstone]

Just killing an afternoon shooting 1/4" dot targets with a pair of AA200's (.22 s200 and a .177 200T).  Not maxed out, want about 50 good shots per fill from either one of them so the energy level isn't impressively high.

 

Really not much to choose from…100% hits on a ¼” circle.

 


It's what I think the little rifles can do bets: stay accurate and provide an impressive number of good shots at moderate energy levels.  Something like 11 foot pounds for the .177 and about 14.5foot pounds for the .22 in order to say in a tight velocity range for 50shots.

Other folks see them as other things...but a powerhouse they will never be.

[cmlynch]

The only TP I can get to or change in any practical way is the barrel port. I took it apart today and it turned out I wasn't quite as open as i thought. I thought I was around 3mm but was actually around 2.6mm. I drilled it out to 2.8ish. I can't remember the actual sizes but I was at a #37 bit and opened it up with a #33 bit.
Oddly, the avg fpe with Monsters fell a little though the max fpe stayed about the same. The curve was steeper and moved about 15 bar lower. Also lost the flatness in the middle I had before. I wasn't overly pleased with the curve shape but like that it moved to a lower pressure.
I then ran a string with Baracudas. They gained max and avg fpe and the shot curve moved to an acceptible starting pressure. 21 shots around 17.5 fpe. 40 shots around 17.12. All 40 inside 4%. This is ok. Not fantastic but ok. The 21 shots are all inside 1% velocity variation.

[cmlynch]

I hunt with mine so I wanted a bit of wiggle room with extra energy/velocity for when I don't know the exact range or am not offered the ideal shot.
14 rabbits and squirrel in the month and a half I've had it.

[cmlynch]

I couldn't see Ribbonstone's pics on my phone for some reason just now. Beautiful guns and shooting. The CZs definitely shoot. I have a CZ452 Ultralux that is a real pleasure.