A 25 Cal Fortitude

[FuzzyGrub]

Over the years, have always wanted to a 25 cal prod conversion.  For many different reasons, over that time frame, always found an excuse not to do it.  It was probably the hardest when Travis built a number of 25 cal mini-rods.   Now that I have three long tube (7/8" OD) guns, it is just natural that they be different calibers.  After allot of contemplation, decided that the newest entry in the collection, will become the "donor".

Here is the baseline starting point: My Fortitude Break-down and mod thread: https://www.gatewaytoairguns.org/GTA/index.php?topic=150000.0

[FuzzyGrub]

The goal for this project is a 25 cal light-weight Mrod.  Power level and shot count should be equal to my original oem synrod.  16 shots at 40FPE 2-3% Es.

Many of you are familiar with the tuning work done by GTA members for Mrods, ie the 40/40 club.  40 shots at 40fpe.  While I never made it to the club, I did achieve 32 shots at 40fpe.  Doing the plenum and air tube volume calculations, it equals 1/2 of the Mrods.  ie 16 shots should be achievable, as long as the porting and other factors can be kept to the same. 

The biggest challenge, for me, will probably be the breech/bolt modifications needed.  Spent a good deal of time thinking about that yesterday.  ie workout the major stumbling blocks, before starting.  I know, what fun is in that?  Don't worry, there is always something you will find on the way, regardless of planning. 

[Dairyboy]

Sounds like a fun project! Well I think you can get a .25 TJs in .4375 OD which would make it easier for getting a barrel in it. The bolt though can't help there

[FuzzyGrub]

The major consideration:

   1) Machine the 1/2" barrel to 7/16"?
   2) Bore the breech from 7/16" to 1/2"?
   3) Combination of both, stepped approach?

I eliminated item 1 primarily from a strength and rigidity aspect, ie the other two options were superior.  This also played into the magazine solution, which I will discuss further down. 

Item 3, was the front runner, because I thought I had to stop prior to the breech screws.  ie the spacing is such that going to 1/2" will break into those screw passage ways.  It would also require taking the mag end of the barrel to 7/16" which again, gets into the mag issue. 

That leads us to, option 2, bore out the whole barrel length to 1/2".   The only mod to the barrel, will be a small notch on each side, where those two mounting screws are located.  On the Fortitude, that is parallel to the TP.


A 25 cal Magazine:

I first looked at modifying a Mrod 22 mag to make it a 25.  The standard 177/22 mag is deep enough for the 25g JSBs.  I thought I could take the clear outer plate and carousal from a 25 cal mag and install in the black casing of the 22.  Yes, I would have to enlarge the exit hole and make the carousal thinner, but seemed doable.   What I found though, was the 25 cal mag is slightly larger ID.  The carousal OD would need to be reduced, and then each of the 8 pellet slots, would need to get deeper.  Other geometry concerns, like the ramp up to the exit hole and the groove that the pellet nose rides in, would challenge my Dremel skills. 

Decided that keeping a stock Mrod 25 cal mag, but machining the breech for the wider mag, would be easier.  ie about 0.100" has to come off of the back of the slot.  I also checked, that there was just about 0.100" with the bolt in cocked position to allow for the wider mag.  Might have to tweak the tip of the bolt probe(0.010"). 

Speaking of the bolt, I have found some stainless tubing to install on the bolt to get to the 25 cal diameter.  This requires a little more work on the breech to accommodate that. 

That's about it for now.  The real mods will not start for a couple of weeks, and will document the process and the successes and any failures. 


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[FuzzyGrub]

Sounds like a fun project! Well I think you can get a .25 TJs in .4375 OD which would make it easier for getting a barrel in it. The bolt though can't help there

Thx Dillon.  I didn't know that TJ had the smaller OD 25 cal barrels.   A 25 GM barrel is already on the way, though.  Plan is to sleeve a 22 bolt with SS tubing and epoxy. 

[mackeral5]

Sounds like a fun project! Well I think you can get a .25 TJs in .4375 OD which would make it easier for getting a barrel in it. The bolt though can't help there

Thx Dillon.  I didn't know that TJ had the smaller OD 25 cal barrels.   A 25 GM barrel is already on the way, though.  Plan is to sleeve a 22 bolt with SS tubing and epoxy.

The MRod barrel makes it easier, assuming you are going to reuse the existing chamber and breech o-ring.  This will be a very cool build.

[screwwork]

John,
The easiest thing to do is turn barrel to fit breech. Your gonna love having the .25 cal Fortitude. I still have my .25 mini-rod from Travis, she's in beast mode with the SS valve installed!

[FuzzyGrub]

John,
The easiest thing to do is turn barrel to fit breech. Your gonna love having the .25 cal Fortitude. I still have my .25 mini-rod from Travis, she's in beast mode with the SS valve installed!

Randy,

Does it use the standard Mrod 25 mag?  I think with a Prod breech, it would need only 0.50" off the back, but also need to cut the slot deeper.  maybe a bushing on the barrel so it snaps in? 

Another way to look at it: If I mess up the breech, it is probably less than the cost of a barrel.   

[screwwork]

John,
Standard mag, no bushing required for the mag. The mag does fit loose in breech but it doesn't fall out!
Also, you'll need to cut a bolt probe slot in the mag facing the bolt for clearance & turn the end of the barrel to fit the factory air stripper if you are going to use factory shroud.

mini-rod breech dims
.704 wide & .839 deep. Hard to measure with scope installed.
 

[FuzzyGrub]

Thx again, Randy.  I already have the 25 cal shroud mount and shroud spacer/stripper.   Those parts are reasonable from crosman.

I'm not sure what you mean about the slot in the back, but also notice the bolt only has a "stub" sleeve on it.  Which means I'm probably not aware of an issue with sleeving the full length of the 0.22 diameter portion of the bolt.   

[mackeral5]

John,
The easiest thing to do is turn barrel to fit breech. Your gonna love having the .25 cal Fortitude. I still have my .25 mini-rod from Travis, she's in beast mode with the SS valve installed!

Randy,

Does it use the standard Mrod 25 mag?  I think with a Prod breech, it would need only 0.50" off the back, but also need to cut the slot deeper.  maybe a bushing on the barrel so it snaps in? 

Another way to look at it: If I mess up the breech, it is probably less than the cost of a barrel. 

I like the idea of opening up the breech and using standard MRod mags, fitting snugly vs. loosely.  Loose fitting mags can in some cases contribute to accuracy problems,

[screwwork]

John,
The .25 cal mag will need a slot cut in it for the bolt probe clearance. See photo of mag. The .22 cal bolt doesn't seat far enough back for the Mrod mag to be placed in/out the breech without the slot in the mag.
I'm not sure why Travis did a short sleeve on the .22 cal bolt? My sleeve is about .328 long, looking inside of the Prod breech I dont see any potential clearance & fit for longer sleeves.

[screwwork]

I have no issues with accuracy, when I do my part with shots at 100yards are fun.
You could make up a barrel sleeve to mag to remove loose fitting mags, make sure it's removable. 
The only thing that is loose is at the .437 barrel to .5 OD mag. You're looking at less than 0.0315 total of clearance side to side when inserting the bolt into the mag, the mag aligns with the bolt and inserts the pellet/slug into the bore. I'm not a bolt slamming kinda guy without any thought of alignment nor did you say or imply that.

[FuzzyGrub]

Ok, I see the slot now.   

I'll have to take more careful measurements the next time I have the Fortitude breech off.

[subscriber]

FuzzyGrub,

I am thinking in the dark here, but some questions will help me make my point:

1. Do the Marauder and Fortitude have the same barrel axis to mag rotation axis, center to center distance?

2. Do the Marauder and Fortitude have the same barrel center to mag floor plate (bottom surface that rests on receiver slot) distance?

If not, you may have to play some games to line pellets up so they feed straight from the mag into the barrel.  One option would be to modify .22 Fortitude mags.  This seems fiddly to me; so this is my proposal:

If my project, I would try really hard to use Marauder .25 cal mags, rather than modify .22 mags.  Due to my lack of knowledge about the Fortitude, I can envision the possibility of needing the barrel axis to be higher in the receiver to accommodate the Marauder mag.  Here is where your barrel OD or frame ID change question becomes interesting:

If the answer to the Marauder and Fortitude questions #1 above is that the Marauder dimensions are larger, then could the Fortitude receiver ID be opened up to .5", while simultaneously eliminating the offset between barrel axis and Marauder mag?

In other words, rather than opening the Fortitude receiver barrel hole perfectly concentric, drift the center (upwards?) to enable the receiver to use the standard Marauder mag.  Now, the top of the Fortitude receiver may be too thin to do this, but my goal is just to plant the idea, for you to ponder.

[subscriber]

 
The only thing that is loose is at the .437 barrel to .5 OD mag.

I would think that having the mag self align with the barrel OD is part of aligning the pellet with the chamber.  This minimizes damage to the pellet, necessary to maximizes potential accuracy.  As such, I think that the mag should match the barrel OD snugly (or flats machined onto it at the rear).

The question remains; what would it take to use Marauder .25 cal mags in a Fortitude?  If the barrel channel need to be opened to .5", then that provides an opportunity to fine tune the resultant mag to barrel axis alignment.

[mackeral5]

 
The only thing that is loose is at the .437 barrel to .5 OD mag.

I would think that having the mag self align with the barrel OD is part of aligning the pellet with the chamber.  This minimizes damage to the pellet, necessary to maximizes potential accuracy.  As such, I think that the mag should match the barrel OD snugly (or flats machined onto it at the rear).

The question remains; what would it take to use Marauder .25 cal mags in a Fortitude?  If the barrel channel need to be opened to .5", then that provides an opportunity to fine tune the resultant mag to barrel axis alignment.

I know that on my Gauntlet .25, when using .25 Mrod magazines i have accuracy issues.  The Gauntlet .25 barrel is 11mm (7/16) so it's basically the same circumstances described here.  The main difference between Gauntlet and Mrod .25 magazines is barrel diameter.   I will say my Gauntlet could use a bit of clean up work in the breech/lead of the barrel so it's not unrealistic to think that with an appropriately massaged breech/lead these accuracy issues may not exist. 

[FuzzyGrub]

The Fortitude uses the Mrod 177/22 magazines.  So those centers and offsets are the same.  It appears that the 25 mrod mags are centered the same.  I opened and edge aligned both and the looked down the pellet exit holes were aligned.  Also measured the bottom of mag to bottom of barrel slot. 

177 mag = 0.170", 25 mag = 0.140", the 0.030 diff is from the barrel radius diff. 

So, think I'm ok there.

FWIW: The Mrod style mags have a recess for the barrel diameter and the mag "snaps in" and aligns to the barrel.  The mag slot cut into the breech, keeps the mag from creating angles, that could cause pellet deformation on loading. 




I also did a more careful measurement on how far back the Fortitude bolt probe is recessed when fully cocked.  It measured 0.075", so given the 0.100" difference, I would need to do the slotted mag.  Thanks for posting that pic, Randy.  It made me check more precisely.   

[FuzzyGrub]

I had the Fortitude breech off, and checked the OD of bolt probe hole, and it will accommodate the larger 1/4" bolt sleeve without modification.  As far as sleeving the 22 Prod bolt, I'm down to choosing between:

1) Smoothbore seamless SS: https://www.mcmaster.com/89895k719

2) Precision (tighter tolerance) welded (and smoothed) SS: https://www.mcmaster.com/6100k345


While probably not needed, considering sleeving the probe for better pellet loading: https://www.mcmaster.com/6100k191

[mackeral5]

Is there any reason to use stainless as opposed to brass? The brass may be easier to  work with. 

I am curious as I have never done this before, and will likely be modding a .22 bolt for use in a .25 in the near future.