A 25 Cal Fortitude

[FuzzyGrub]

Well, so far, the accuracy is showing much better than I would have expected.      In my 10yd basement range it is hole in hole, and what I would expect from the 25 GM barrel.  The fps is down about 60fps, but can't say that is the choke.  This is a reg,ed gun with 0.187 ports, but the barrel port is stock, since I might be returning it.  The wind is too strong and gusty, to test at any further distances or check for spiraling. 

[FuzzyGrub]

The major thing that this whole build relied upon, was the boring of the breech from 7/16" to 1/2".  I had never done this and had not seen others describe how to before.   The key to "my" process was a step drill.  See below.  It has 1/16" increments with 1/2" as the last step.  My thought was that the 7/16" would be the pilot for the 1/2" step.  There is about 1/2" length of the 7/16" diameter.  That 7/16" step is a tight fit though. 

Having only 1 Fortitude breech, and crosman hasn't released the schematic yet, did a small test on a Prod breech.  I have a 17" LW barrel that was machined for the shorter Mrod breech.  Instead of machining the barrel, decided to do a 1/2" test bore.  Seemed to work fine. 

Note: The LW barrel is 0.470" and will add a brass sleeve.  I'm starting to really like this brass tubing.   

[FuzzyGrub]

Now, I did this on my small bench top drill press.  It was lacking in enough height adjustment to do this in a few steps.  I had to reposition and re-align numerous times.  A full size drill press would have been much better.  The breech was aligned using the 7/16" drill, and needed a couple of short extension for the step drill.  The step drill was too long to do the full depth, or it would damage the back side of the breech.  I  used a 1/2" to do the final inch or so.

NOTE: One thing I forgot to mention: Using the step drill for a piloting drill, requires taking the cutting edge off of the piloting step.  For this case, I took a file to the 7/16" dia step edges.  I also put some oil in the breech hole.  The point is, you want it to follow the existing hole and not try to cut its own path. 

[screwwork]

You're a braver man then I, from my old machine shop days it tells me to bore it out the breech using a 4 jaw chuck in a lathe to keep the barrel to breech aligment correct. You did it in a drill press, congrats.

[mackeral5]

We all have our series of operations that best equipped for and most comfortable with.  Very nice demonstration here. 

I will likely have to increase breech size sometime in the near future, thanks for sharing the unibit idea.   

[FuzzyGrub]

from my old machine shop days it tells me to bore it out the breech using a 4 jaw chuck in a lathe to keep the barrel to breech aligment correct.

Yea, if I had one of those.....     I thought this was worth a try.  So far, don't think I need to call David, to bail me out.   

The proof will be once I scope it up.  But, already eyeing some other prods, such as my 2201 pistol.   

[FuzzyGrub]

I just had to do it.  Launched the first dozen pellets through her.     Be it with the existing porting and only going 650fps.   

[subscriber]

Impressive substitution of skill, improvisation and determination; over a lack of equipment.

[mackeral5]

I just had to do it.  Launched the first dozen pellets through her.     Be it with the existing porting and only going 650fps.

Not a bad place to start.

[FuzzyGrub]

After allot of thought about how I could do the mods, decided to go with modify the mag, for now.   Machining the breech would have been tedious with only files, blocks/sandpaper, and likely to slip and scratch/gouge something in the process.  Keeping nice and square and even measuring would be rather difficult.  Machining the breech is the better answer, but will send that out to have a pro do it, sometime in the future.  I will probably have them add the two mounting holes so it could be installed on a Prod, too.

So, I starting with the standard 25 cal mag, and basically make it as thin as a standard 177/22 mag.  That involves sanding the black outer case and the inner revolver wheel.   I have the rough sanding done to both, and within 0.010" of where they need to be.  I should complete the finish sanding, massaging, and testing this evening.  This seems well within my capabilities, but is time consuming.  Will post pictures and specs this evening. 

[mackeral5]

I look forward to seeing the pics.  I may go that route on the next one. 

I wondered about fitting the .25 wheel into a .22 mag???

[FuzzyGrub]

Mag Modification:

The 25 cal Mrod black shell is 0.570” wide and needs to go down to 0.470" of the standard 177/22 mag.   I flat sanded it using 80 grit to do most of the work.

Procedure:
•10 CW circles
•10 CCW circles
•Rotate 180 degrees
•10 CW circles
•10 CCW circles
•Measure with calipers
•Adj as necessary
•Repeat until 0.005” above final spec
•Drop to 320 grit to final spec
•Polishing 400 then, 600 grit


Note 1: The dimple that holds the mag open for the 1st pellet is eliminated

Note 2: Epoxy fill is used in protrusion to prevent hollow out

Note 3: the thicker material at the protrusion would often be wider than the rim.  The “adj as necessary” required some extra pressure on that end to equal out.

Note 4: Before going to 400 grit, break the sharp edge around the inside.


The 25 cal revolver cartridge is 0.450" wide and needs to be sanded down to 0.350".  I used same process as above, but did not find that any adj was needed.  There are 6 tabs on the inside of the revolver that need to be sanded down flush. These are spring stand-offs that are not needed for a thinner mag.   I used snippers and exacto knife to eliminate them.

There will still be some of the brass left for the center screw.  The sand flashing can be pushed through with small brad, then screw should thread in. 

The clear face-plate was not modified.

The barrel side of the mag was not modified outside of epoxy fill.

Spring and screw unmodified.

 

Other good mod info on mags: https://www.airgunnation.com/topic/umarex-gauntlet-tweaking-the-magazine/  Another good video from Hajimoto

 

[mackeral5]

Getting closer.  I assume due to available plenum/storage you are designing this around 25gr ammo?  If so then no need to ever worry about modifying the breech?

[FuzzyGrub]

Getting closer.  I assume due to available plenum/storage you are designing this around 25gr ammo?  If so then no need to ever worry about modifying the breech?

Yes, the 25g ammo and 40ish fpe, and a 16 shot targeted count.  Heavier ammo to go with higher power, would be better to use a different platform.  That said, I just tried some 34g MKII in it, and cycles fine. 

I am going to find a way to add that cover locking nub back.  Maybe a tiny brass brad. 

[FuzzyGrub]

Valve Plans:

•Open exhaust to 0.187”
•Open poppet pocket to 0.230”
•Change to mrod/prod poppet
•Possible mod of inlet washer
•Possible change to conical spring
•Deepen trigger assy 3rd screw relief
 

Plenum has landed on the east coast and should be here soon.

SS washer for plenum to plenum interface, ľ” OD sanded the OD to fit easily in air tube
 

Other to dos:

•Set reg to 2kpsi
•Make 0.187” Poly TP
•Barrel: modify flat and add shroud mount flat
•Bolt: final bevel check on brass sleeve and seal

[mackeral5]

That's surprising to see 34gr fit and cycle.  Very cool!

[FuzzyGrub]

I did find a small brass brad and cut the head off, and then rounded it off using a drill as a lathe and shaping with sandpaper.  Drilled a hole thru the clear cover using a drill bit that was 0.005" smaller diameter than the brad.  Using that as the pilot, drilled the hole in the epoxy.  Simply pressed the brad in place.

In the second picture, the pellets are the 34g JSB MkII's.  You can see they sit a few thousands below the top of the revolver wheel. 

The mag snaps in, nice and secure. 

[mackeral5]

I haven't looked very close, but I assume you have.  What prevented you from using a thinned .25 wheel in a modded .22 body?

[FuzzyGrub]

I haven't looked very close, but I assume you have.  What prevented you from using a thinned .25 wheel in a modded .22 body?

The 22 body is designed to snap in to a 7/16" dia barrel, not the 1/2" GM.  You could machine the barrel end down, though.   The 22 is also a slightly small inner diameter of the black case, that may have caused some mods to each of the pellet slots in the revolver.  The circular channel where the pellet head rides looks slightly different.  I'm not sure if those last two items are real issues, or just worry beads.  The 22 exit hole has to be enlarged, but in an offset way with the loading ramp.

Basically, I thought using a single 25 mag was easier with less risk, and didn't expend two different mags. 

 

[mackeral5]

I haven't looked very close, but I assume you have.  What prevented you from using a thinned .25 wheel in a modded .22 body?

The 22 body is designed to snap in to a 7/16" dia barrel, not the 1/2" GM.  You could machine the barrel end down, though.   The 22 is also a slightly small inner diameter of the black case, that may have caused some mods to each of the pellet slots in the revolver.  The circular channel where the pellet head rides looks slightly different.  I'm not sure if those last two items are real issues, or just worry beads.  The 22 exit hole has to be enlarged, but in an offset way with the loading ramp.

Basically, I thought using a single 25 mag was easier with less risk, and didn't expend two different mags.

Perfect explanation, thank you very much.