Reducing OD of BNM Breech Bolt probe. I need advise!

[Rdsail]

I have been investigating the loss of 90fps on my disco after I added the BNM breech. I noticed that the bolt probe obstructs about half of the barrel port.

Here is a picture of the the blockage:



Has anyone removed material of the probe to make open up the probe?



I could remove material so the probe is longer. If you look at the picture I can take it back to were the bevel starts. I don't think this will cause any problems but I was hoping that someone has done this before and if they had any results.

I can probable reduce the OD of the probe tip by about .5mm or a little lest and bring it back about 3mm.

 

[rsterne]

I can't tell from the photo what part of the bolt/probe is obstructing the port with the bolt fully closed....

Bob

[Rdsail]

If you look at the bolt. You would have the probe to the right (which is a thick probe) and to the left
Of that there is the beveled edge of the bolt. The whole beveled part covers half of the barrel port. I don't think it will cause any problems removing 2 or 3 mm from the bolt. I pushed some pellets through it and the probe does not need to be any longer because the pellets
Clear the port.

[Rdsail]

I can't tell from the photo what part of the bolt/probe is obstructing the port with the bolt fully closed....

Bob

I hope this picture is better. It took in the same orientation as the picture of the bolt able.

[Rdsail]

Well, I decided to proceed. I took measurements I the mod did does not look like I would cause any problems.

Here is the original picture or the rifle port. you can see that it is block more than 50%



Here is the new picture with the material removed



Now here is the original bolt.



Here is the bolt with the material removed. The probe was 1/8 in thing and now it is .1 in thick. the tip was hollow and the wall on the tip was very thin so I added some JB weld to it. Right now it extends too much. I'll be sanding it back when it cures. I might extend it .5 or 1 mm if it does not interfere with the magazine.



I will not be able to test this until I get from crosman the stock slug screw.

The blockage on the barrel port should explain why adding hammer preload did not had much of an effect on the speed. there was but so much air that could go through the port due to the +50% of the port being blocked. This should not be a problem with application that use low FPE.

[breakfastchef]

The BNM bolt body occludes around half of the transfer port and the bolt tip is quite beefy. Lots of turbulence and such.



I have done a modification on the BNM bolt. I would not recommend trying to re-profile the OEM bolt probe because the tip is hollow. Get too thin and the tip is weak and useless. Lose the tip and your pellet won't seat past the transfer port. I chose to cut off the bolt to the transfer port, drill out the face and insert a 3/32" rod to act as an extended probe. This modification brought back @40 fps to a Crosman 1377 carbine build I was working on. Simply reducing the .18" diameter of the thicker part of the bolt was something I had considered, but likely would not have regained as much fps as the extended probe modification.



The above modification works great with a single shot tray. But, you do run the risk of losing some accuracy when using a magazine. As you can see below, the modified bolt begins pushing the pellet out of the magazine well before the thicker part of the bolt properly indexes the magazine's internal rotary tray with the exit hole. This situation results in more scraping of the pellet as it is chambered.

[Rdsail]

Breakfastchef,

Nice to see you here in GTA. I stated inspecting everything after you post in the yellow. The only difference on mine is that I did not add a rod I just used the one there. I might eventually cut it and add a 1/16 rod. to make it even smaller. I made my prove smaller than yours. Just enogh to clear the port

Mine was obstructing 3mm of the port. My disco is set for high flow so that blockage is a big issue.

Thank you for the help

[PakProtector]

hey-Hey!!!,
The part of the bolt forward of the back side of the transfer port can go away with a grinding wheel. As long as you make the edges smooth, and don't impinge on the sealing o-ring you can remove lots of it.
cheers,
Douglas

[breakfastchef]

hey-Hey!!!,
The part of the bolt forward of the back side of the transfer port can go away with a grinding wheel. As long as you make the edges smooth, and don't impinge on the sealing o-ring you can remove lots of it.
cheers,
Douglas

And, you need to break the newly created edge so it enters the magazine smoothly.

[Rdsail]

doug,

I kept mine short because I don't want any more volume/area for air to pressurize.

breakfastchef,

I'll brake the edge next

[breakfastchef]

Glad something I posted was of some help. You are the only other airgunner I know of who also experienced/noticed this same loss of power. Unclogging the transfer port area definitely helps. There are still areas of velocity loss that are directly related to the design of the breech.

Also, let Sergio know you have this problem and see if he will send you and extra OEM bolt, and a selection of longer transfer sleeves. I found the .480" high transfer sleeve set better in the barrel transfer port.

Breakfastchef,

Nice to see you here in GTA. I stated inspecting everything after you post in the yellow. The only difference on mine is that I did not add a rod I just used the one there. I might eventually cut it and add a 1/16 rod. to make it even smaller. I made my prove smaller than yours. Just enogh to clear the port

Mine was obstructing 3mm of the port. My disco is set for high flow so that blockage is a big issue.

Thank you for the help

[Rdsail]

I'm glad you posted that. My disco can shoot up to 40 fpe and now it is maxing at 29. I need every bit of port to make it shoot fast and the bolt is strangling everything.

[rsterne]

My question is.... WHY would anyone design a bolt that blocks off the transfer port?.... Make NO sense to me....

Bob

[Sqrl Klr]

There are still areas of velocity loss that are directly related to the design of the breech.

How so? Interesting thread. 40fps gained so far is indeed a slight improvement. Will be interesting to see the total fps gained when your mods are done.

[Rdsail]

I hope I'll be testing by the end of the week. I email BNM about getting longer transfer ports. I was hoping to get a couple with the both ends a little longer. The bottom 1 mm and the top 2mm. that way I can sand off material to make a good fit. This might come later though.

[breakfastchef]

I gave up on the BNM system and sold it off to a fellow GTA member who is thrilled with it.

The bolt probe is without a doubt the piece that kills a huge amount of velocity. The other areas of possible velocity loss in the BNM breech include the longer transfer sleeve and the air space behind the transfer port to the breech seal. You know very well about the rear o-ring  (wink, wink). I am guessing that as air travels up the sleeve and hits the transfer port of the barrel, it goes in two directions. All this extra air space needs to be pressurized and may also cause disruptive turbulence. So, if some of the pressurized air that propels the pellet also has to go backward, you are losing potential velocity.

I also found the transfer tube I received with my BNM breech was a little too short. After Sergio sent me a few sleeves, I found the longest one made a better connection with the barrel. This resulted in some velocity gain.

I am not bashing the BNM products. They are well made and designed to be an easy bolt-on device that provides the airgunner with multishot capability on the Crosman platform at a reasonable cost. I did supply Sergio with all my data and he thought that a flow-though bolt would possibly solve a large part of the velocity loss. I never did follow up with him on this, but it seemed like a very good solution without reworking the whole design.

There are still areas of velocity loss that are directly related to the design of the breech.

How so? Interesting thread. 40fps gained so far is indeed a slight improvement. Will be interesting to see the total fps gained when your mods are done.

[Rdsail]

Finally got my parts in and have some results.

First a few things I did:

Use the disco hammer instead of the Prod: There was no gain in power by doing this and the disco hammer makes a lot more noise when striking than the Prod hammer. I put the prod hammer back.

Bolt modifications: I remove material on the bolt so it would not obstruct the barrel port and reduced the probe to .098". this is as far as I could go due to the the probe tip is hollow. The probe on the disco breech that I had modified is .051" so much smaller. Doing this mod did not change anything on the mechanics or ease to load the pellets.

Results: I had an increase of +35fps by modifying the probe. My range on adjustment is much higher now. It tops out at 885 FPS with about 3/4 of a turn of preload there which seams that I still have to allow for a little more air flow. As of right now I'm going to leave it at 870 fps. It uses a lot less air at this setting. Which is about 1 1/2 turns from binding.

One thing that I notice is that when I max out at 885 and I add more preload, my t port leaks a little air. I contact BNM last week to see if I can get a longer tport that I can sand down to allow for a better fit. I feels to me that the tport is not putting enough pressure on the breech gasket and the barrel port or not at all.

The accuracy is superb. I'm only shooting from 20 yards but one ragged hole is all I get with an occasional flyer about 1/4" off target.

Hopefully BMN send me the new tport soon. So I can test it out.

Last option: Cut the probe off and attach a new probe that would be 3/32" thick. to allow for less air restriction.

[Rdsail]

I run a full string from 3000 psi to 1300psi.

results from 3000 psi to 1600psi - JBS 18.1

High: 877 (31 FPE)
Low: 871 (30.5 FPE)
Ave: 873.6 (30.7 FPE)
SD: 1.58
Extreme Spread: 6 FPS or 0.69%
Shots: 60

Once I pass the regulated pressure.
The gun picks at 883 and goes down to 871. The it takes a nose dive.
This is for about 15 shot.

My efficiency when down some with the new breech but will 60 to 75 shots I don't complain. I had a drop of about 2 FPE with the breech. Hopefully the new Tport I'm getting will gain some of that.

[breakfastchef]

Ask Sergio to make a flow through bolt for you to try. The flow through design would preserve the necessary bolt tip profile to work with the magazines, and would eliminate solid parts of the bolt/bolt tip from filling up the transfer port.

[Rdsail]

IMO the flow through bolt actually block more air than the probe. I did test with the disco breach and tried to open flow though areas as much without the it turning to weak and still a very thin probe (3/32) performed much better. The problem with the flow through bolt is that restricts the air flow at the barrel port and then chokes the airflow as it goes through to the pellet. This is coming from practical experience. The best scenario would be a probeless bolt that retracts after you put the pellet in.