Canadian Airgun Forum

The #1 Community for Airguns in Canada!
It is currently Wed Nov 20, 2019 7:57 pm

All times are UTC - 5 hours


The Canadian Airgun Forums are a place for people to discuss and learn about airguns and the airgunning sport in Canada. There are lots of discussions about airguns, airgun accessories, reviews, modification and repair information, airgun events, field target and free classifieds!

 

You need to register before you can post: click the register link to proceed. Before you register, please read the forum rules. By joining our free community you will have access to post topics, communicate privately with other members (PM), respond to polls, upload your own pictures, and access many other special features. Registration is fast, simple, and absolutely free! To start viewing messages, select the forum that you want to visit from the selection below.








Post new topic Reply to topic  [ 11 posts ] 
Author Message
 Post subject: 1377 velocity question
PostPosted: Mon Sep 07, 2015 6:57 pm 
Offline

Joined: Thu May 07, 2015 8:30 pm
Posts: 1186
Location: Eastern Townships
I'm back on my 1377 project, and I'm about to replace the plastic breech with a custom bronze one. The new breech will have the barrel sitting 1/4'' higher than the plastic breech, so I'll have to use a 1/4'' longer transfer port.Will I be losing some fps by doing so? Should I make the new transfer port ID smaller to keep velocity up to its original level? The original TP ID is 0.125''.

Also, I have an M4-177 barrel sitting useless that I could put on my 1377, a bit more than 16'' long. What length should it be if I want to stay under 500fps? I understand I'll have to re-drill a TP hole in the barrel to use it on my 1377, and also make a taper entry for the bolt.

Thanks in advance for your help!


Top
 Profile  
 
PostPosted: Mon Sep 07, 2015 8:18 pm 
Offline
User avatar

Joined: Sat Nov 03, 2007 4:19 pm
Posts: 8979
Location: Coalmont BC
You may, in fact, have to enlarge the transfer port diameter slightly, not make it smaller.... You are interested in the pressure and volume of air delivered to the back of the pellet, not it's velocity....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


Top
 Profile  
 
PostPosted: Mon Sep 07, 2015 11:39 pm 
Offline

Joined: Sat Jan 25, 2014 9:16 pm
Posts: 1271
Location: United States
Since the gun is capable of much more than 500fps I'd say what little you lost could easily be brought back up by reducing the power robbing CA mod. Power is dropped with bleed hole in that gun right? So I'd just make it smaller until you kiss the legal limit.
I think I'd prefer to have the pump and valve modded like how you do them for max power, but then by reducing the volume of the valve assy you could drop the power to legal. Doing that would mean 500fps in far fewer pumps, maybe even five pumps?
That long barrel is an option I suppose, but if limited to 500 I don't see the point. Kinda takes the pistol bennies outta it, no more putting it in your pocket etc.
A bronze breech you made yourself? If so pix please...


Top
 Profile  
 
PostPosted: Tue Sep 08, 2015 9:03 am 
Offline
User avatar

Joined: Sat Nov 03, 2007 4:19 pm
Posts: 8979
Location: Coalmont BC
Modding the gun to achieve 500 fps in 5 pumps will certainly make it illegal at the 10 pumps Crosman recommend, should it ever be tested.... bad advice....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


Top
 Profile  
 
PostPosted: Tue Sep 08, 2015 11:18 am 
Offline

Joined: Tue Oct 13, 2009 1:27 am
Posts: 2514
Location: Vancouver
Funny, my memory of the instructions with my little brother's 1377 back in 1989 was that they recommended no more than 6 pumps. Something about damage becoming likely if you pumped it more than that. But perhaps memory is faulty...


Top
 Profile  
 
PostPosted: Tue Sep 08, 2015 8:01 pm 
Offline

Joined: Sat Jan 25, 2014 9:16 pm
Posts: 1271
Location: United States
If the chamber volume is reduced as described then peak possible pressure would achieved in fewer pumps, but peak cannot be exceeded because that's all the pump can make until you improve the pump.
A flat top plunger setup will allow more pressure, which is the better way to make power, but peak pressure on those plungers is adjustable if needed. So you could block the bleed port to increase efficiency, then by controlling the peak pressure and chamber volume you could dial it in to make 500fps in the least pumps possible. If think 500 at 5 pumps it possible but I haven't tried it, but 6 or whatever. If being legal requires 10 pumps then dial it in for that. If the balance between the chamber size and pressure is right I think you could set it so 10 pumps is legal, but 5-6 pumps is close enough to full power than there's no need to exceed that. Just like an oem full power makes more power on 15 pumps, but it's not much and not worth it so few people exceed 10. Make sense?


Top
 Profile  
 
PostPosted: Tue Sep 08, 2015 8:17 pm 
Offline

Joined: Thu May 07, 2015 8:30 pm
Posts: 1186
Location: Eastern Townships
Thanks a lot for the enlightenment, Bob. I'm still a novice in that ''department'' :oops: . I was thinking about using a stainless steel 5/16'' bolt as material for the new TP, maybe drilled to 9/64''? (I use my drill press as a mini lathe).

Chevota, as usual, it's very nice to get replies from you :wink: :) . I guess I'll stick with the stock barrel and try to improve the valve like you suggest, I like the idea of a bit more pressure by reducing the inner volume of the valve, but I'm also wondering what velocity a flat top piston would yield in an otherwise stock pistol. Having an adjustable piston would make it easy to adjust the velocity, it does make a lot of sense! Maybe it'll need some tweeking of the hammer and spring as well, but I'll see. I planned to buy a Chrony this week, so I'll be able to test it thoroughly. As Bob mentioned, I have to keep it under 500 fps at ten pumps. As for the pix, sorry to tell you that, you'll have to wait a bit. Yes, I'm making it, but it's not finished yet! The barrel hole is drilled, but I still have to hone it to size, and I have to make the bolt and also figure out other details :hsmash . But it's coming!

Gerard, thanks for your input :) . I really like to read your posts on hammers and springs mods, if I use a flat top piston and valve, I guess I may have to lighten the hammer and shorten the spring. Can't wait to get that Chrony!


Top
 Profile  
 
PostPosted: Tue Sep 08, 2015 8:38 pm 
Offline

Joined: Tue Oct 13, 2009 1:27 am
Posts: 2514
Location: Vancouver
Yeah, it really does seem that for most of our tunes a lighter hammer is the way to go. As others have said a number of times... it's almost as if Crosman has something invested in wasting too much CO2, hence the heavy hammers. But of course with the 1377 there's no CO2 involved, just happens to use some of the same parts as the 22xx series.


Top
 Profile  
 
PostPosted: Tue Sep 08, 2015 10:38 pm 
Offline
User avatar

Joined: Sun May 20, 2007 10:26 pm
Posts: 324
Location: Any Town ONTARIO
I did not think a 13xx hammer was the same diameter as a 22xx gun . At least mine are not .


Top
 Profile  
 
PostPosted: Tue Sep 08, 2015 10:56 pm 
Offline

Joined: Tue Oct 13, 2009 1:27 am
Posts: 2514
Location: Vancouver
Goes to show you I'm still a noob. Oh well.


Top
 Profile  
 
PostPosted: Tue Oct 27, 2015 4:27 pm 
Offline

Joined: Thu May 07, 2015 8:30 pm
Posts: 1186
Location: Eastern Townships
Just a little update. I finally found out the original transfer port ID, it's 0.086'', or drill #44. I bought that 1377 lightly used, supposedly with no mods. Seems there was at least one...At least, the valve and piston are untouched.


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 11 posts ] 

All times are UTC - 5 hours


Who is online

Users browsing this forum: No registered users and 1 guest


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
News News Site map Site map SitemapIndex SitemapIndex RSS Feed RSS Feed Channel list Channel list

Powered by phpBB © 2000, 2002, 2005, 2007 phpBB Group

phpBB SEO