Custom Benjamin MROD Bullpup *NEW* 15 Round Mag

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Continued from above.

Next step was the cover.

I started again with cutting a piece of 3mm thick acrylic down to my desired size. I then fastened it to a piece of aluminum with some amazingly strong and thin double sided tape (again from McMaster)

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First program just drilled the holes and cut out the slot, I try to only do light machining with only tape holding it down. Once it was finished I drilled and tapped the aluminum in the hole in the center of the magazine cover. And fastened the part down with a screw for a little bit more security.

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Second program finished off the outside. All that was left was to turn down a pin on my lathe and press fit it in and drill a recess for the locking pin on the magazine housing. Top one is fresh off cnc, bottom one is finished.

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Next step is the inside piece that holds the pellets. I was lazy and the small slot in the underside of the part I could not make with the tools I had, so I just 3d printed it, it came out quite nice.

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The strange shaped black part on the bottom of the inside of the 3d print is called a raft( it has to be removed) Its used to create a level printing surface for the 3d printer, witch is basically a really fancy hot glue gun in my opinion.

After cleaning up the print I put all 3 parts together and they fit well together.

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Unfortunately all things needed to be mirrored for this to work, including the factory left hand torsion spring.

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I couldn't find anything even close to working for this spring. So I ordered some spring wire (from McMaster again) and made my own on my lathe. Ill post the process soon.

A couple more pictures of all my new free cheek space with the springless left handed magazine.

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[35remington]

The use of the 3D printer in this process is just unreal. Great work.

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I got in some spring wire today. Got a couple sizes and materials so I could experiment with. Ended up going with a carbon steel wire .022" in diameter. There is a learning curve with making springs, it took me a couple tries to get it correct.

Here is the method I used. The rod with a hole drilled in it is to pass the spring wire through, and bend the end so it wont come out. The rod is clamped in the vice chuck. The rod clamped in the tool holder with the slot in it is to guide the the wire as it wraps around the other rod. I set the threads on the lathe to 32 threads per inch. It gave results I was looking for. I then rotated the lathe chuck by hand, I only used a foot of wire at a time, never used the entire role for safety.

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Here we go, a little hand stretching to get it perfect and its ready to go, on right hand spring.

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And a couple pics with a loaded magazine. I put 10 rounds through it with no issues. Pretty happy with the results. Im going to have to machine the inside piece out of acrylic also though. The 3d printed part is just not up to my standards.

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[caniborrowsomeammo]

Amazing. I'm a lefty too and a BP wasn't in the cards for me because of the mag. You sir are doing some great work and your presentation is to be envied.

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All right, now the trigger assembly.

This is my first attempt. It was made up of 4 main pieces, 7 pieces including the two air tube clamps and scope rail, I did this to keep it modular and if the front or back trigger assemblies need changing I would only have to redo part of it.

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When designing it, I did not not want to reuse any of the mrods original trigger like I've seen in other designs. But I did use a similar catch mechanism as the mrod. I did change the type of safety mechanism, thought it would be easier for me to produce. All you have to do is slide the safety trigger back to lock the trigger in place.

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Unfortunately. In my excitement to design, I didn't make sure the stock fit me. This assembly is closer to fitting a child than me. Good news tho, the trigger and catch mechanism worked very well with a little modification.

So here comes revision 2. I kept most the same but consolidated 3 of the parts into "unibody" design. Show in green below. I changed to safety mechanism to to an even easier to produce version. It fastens to the mrod upper in 4 places, two on the air tube with the clamps, and two screws into 2 of the factory holes. No modification to the factory air tube was necessary.

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I also didn't like all the springs used in the original mrod design. I consolidated down to 3 springs even with the new bullpup linkage. The spring on the linkage can be adjusted with the shaft collar, it controls the pull force of the trigger. The aluminum cylinder with two screws coming out of it is to add adjustability to pull the slack out of the bullpup's linkage.

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Here is the insides of the front trigger assembly. The front screw (closest to trigger) is for adjusting the the first stage of the trigger. the back screw is for the second stage. And it is set very short and light right now.

Safety off
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Safety on

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Be prepared my terminology is probably not correct below.

The back catch assembly, is very similar to how the mrods factory catch works. When you pull back the bolt, it pulls back the striker weight. Once it is pulled back past the catch, the catch pops up in front of the striker weight. When you put the bolt back forward the striker weight pushes the catch forward (a slot in the catch allows it to move back and forward) and it is pinned on the "catch stopper" when you pull the trigger it pulls the "catch stopper" forward releasing the striker weight and firing the gun. The "catch stopper" spring needs to be pretty strong for this type of mechanism.

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Loaded
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For prototyping all the innards are made from 6061 aluminum. All the innards will be converted over to a wear resistant stainless steel for longevity. The current aluminum parts have over 1000 shots on them with no misfires or issues, but they are on borrowed time.

[AwesomeAnt]

Wow nice work! Looking forward to see your results.

[RayK]

Nice work! ANd thanks for sharing your journey.

These are the standard names for the parts you mentioned
"catch stopper" = sear
"striker weight" = hammer

Ray

[screwwork]

Nice build and photos, you make me miss having access to a machine shop.
The clear mags in left and right hand for the Mrod can be a good product to make and sell to the right crowd.
Due to labor intensive work involved to manufacture it would cost more but it would be bag ass and some people are willing to pay for that.
For the spring forming you could make a simple guide and drag block that would mount in your tool holder to have a constant pressure on the spring wire.
We used simple drag blocks made from out of electrical grade fiberglass (GPO3) for tensioning the copper conductor for winding electromagnets on to the winding machine. Anything not plastic or non-ferrous should work due to the heat from fiction. Wind your spring on to our form making a longer spring and cutting it to size may make it easier than winding exact lenght.

[scharf]

Nice build and photos, you make me miss having access to a machine shop.
The clear mags in left and right hand for the Mrod can be a good product to make and sell to the right crowd.
Due to labor intensive work involved to manufacture it would cost more but it would be bag ass and some people are willing to pay for that.
For the spring forming you could make a simple guide and drag block that would mount in your tool holder to have a constant pressure on the spring wire.
We used simple drag blocks made from out of electrical grade fiberglass (GPO3) for tensioning the copper conductor for winding electromagnets on to the winding machine. Anything not plastic or non-ferrous should work due to the heat from fiction. Wind your spring on to our form making a longer spring and cutting it to size may make it easier than winding exact lenght.

Yea, other than the r&d time, the machining is by far the most costly. I really want to experiment with urethane casting. The right material can be very close to an abs injection molded part and could also be made in clear. If I did that then it could be feasible to make the parts relatively cheap. But as it stands guessing people are not going to be willing to pay $75 for a clear mag haha. And thats if I work for cheep. Thanks for the suggestions on the springs, im starting to think of it as an art form, I find it strangely interesting.

[scharf]

Alright, now here is something I'm pretty happy with. As some have seen from my previous posts I made a clear left handed mrod magazine. But copying designs is not my style, I enjoyed making it but felt like I needed to do more. So, I designed my own, 15 round magazine. It uses some of the factory ideas, with a couple of my "improvements"

The outside housing and clear cover were made with the same procedures a shown in the previous posts. The center pieces that holds the pellets was made with a different method. I made 3 pieces out of 6061 alumium, acrylic, and I believe some sort of ABS (not 100% sure but it machines really nice).

1st. I cut a piece of material down to my desired size and drilled 4 holes in the corners for mounting. Next I drilled the same hole pattern along with 4 other holes (get to them in a couple of steps) in a piece of aluminum and tapped all 8 holes with out removing the part from the vise. I then fastened the material to the aluminum block with the 4 outside holes.



2nd. I wrote a program to cut out all the inside features of the part. The key to this method for best accuracy is the used the same CPL(0,0,0 reference for the cnc) of the mounting block as I do with this program and just account for height of the material. By never moving the block from the vise and using the same CPL as the mounting block it takes probing the part out of the question allowing for the best accuracy.



3rd. I fastened the 4 screws in the inside of the part (from the other 4 holes above) And then removed the 4 outside screws. This method allows me to machine around all sides without having to ever move the part.



4th Run the last program to cut around the outside features of the part and be amazed. As the part turned clear!!! Just kidding forgot to take a picture of the gray parts last step.



And some pictures of the finished aluminum part. And some explanation to what I consider improvements.






Improvements over stock.

1. I added a full circle lip around the top of the inside piece. The hole is just barley big enough to let a perfectly circle pellet skirt through. If the skirt is damaged or misshapen it wont go trough the hole. If it is skirt shape is slightly off the hole will reshape the skirt to its proper shape. (A very small bevel on each hole keeps the aluminum from removing material from the pellet.) This eliminates my need to sort out pellets as much. I have yet to have a stray flyer with this magazine.



2. The extended the center hole down with a shaft for 2 reasons.(seen below in center of part)
A. The factory hole only makes contact on the center shaft with a thin section of material. The extended shaft takes out a lot of play in the magazine, allowing for more consistence rotation, more consistency could lead to better accuracy.



B. The factory spring, as it tightens can rub on the stationary shaft of the magazines black housing. The on and off friction can cause inconsistent torque. The extended shaft only allows the spring to rub on the moving shaft, allowing for more consistence rotation, more consistency could lead to better accuracy.

3. I shortened down the inside geometry to only accept .3" and shorter pellets. It is designed around all my favorite pellets, JSB Heavy, JSB RS, H&N Sport Field Target and Crossman ultra magnums. All these fit perfectly where the skirt of the pellet is always held perfectly center by the back full circle. Again allowing for more consistency.

4. I tightened up the hole spacing,refined semi circle geometry and made the stop part smaller so I could get more pellets in a smaller diameter than if I would have used the same method as the factory mag.

More pictures of the finished parts and magazine below in next post.