Does anyone know of a source for headspace gauges for the 20x102mm Vulcan? I've looked pretty extensively on the internet and have come up with no leads. Perhaps Forster or Clymer could make them on a custom basis, I don't know, I haven't made an inquiry with either company yet.

If there aren't any gauges out there, how do people who construct 20mm DDs set headspace?

Thanks for any assistance y'all can provide.

Mine is singleshot. Used factory steel dummy round, tightened barrel until snug, tightened locking.

Thanks, I kind of figured some folks were using dummy rounds.

I also used some live factory rounds to verify it was correct as well.

All I have shot are reloads using new brass, so they all fit fine. Since it's singleshot, reloading is easy and the brass is already sized to my chamber.

I have a set of gauges and both a roughing/finishing reamer for 20x102mm from Manson Reamers. It wasn't cheap, it was a long wait since it wasn't something he keeps on the shelf, but it was worth it as I deal with 20x102mm pretty regularly. I don't know if he still produces them, but it was a lot more expensive than the .50 BMG reamers and gauges I bought. JGS will custom make you a headspace gauge as well.

CH4D also produces the reloading supplies, which you will really want unless you like spending $14 per shot.

Not to hijack this thread, but I'd love to see pics of everyone's 20mm's.

It's a reasonable request, and I mean no offense by this, but I tend not to post pictures of my firearms online. It's just a habit, but I hope others are willing to share theirs. Alternatively, you can find images of 20mm rifles all over the internet if you'd like to see the wide variety that have been made.

Thanks for the references. I spoke with Dave Manson and he doesn't offer headspace gauges in 20x102mm now. I sent an email to JGS Precision and am awaiting a response. I also did some more research and found that Pacific Tool and Gauge may be a possible source and am waiting for a return call from them.

Yep, reloading is the way to go for this and I've been accumulating components for more than a year now. I've got plenty of WC872 powder and cases, and a fair supply of projectiles. It's the projos I most concerned about in the long term. The two sources I was buying from have dried up completely and the remaining suppliers I know about have bumped up their prices quite a bit. I guess the well is beginning to go dry on the surplus stuff.          

If you're seriously wanting to get into shooting 20mm a lot, it may be cheaper to purchase a CNC lathe, bar feeder, and copper or aluminum bar stock and work up loads from there. I hardly have a year where I shoot more than 100 rounds, so I have a lifetime supply of projectiles already or I'd set up my CNC to turn a bunch out for myself.

I received prompt responses from both JGS and Pacific Tool. No luck with JGS, since they aren't making tools for anything larger than .50 caliber, other than for shotguns. Surprisingly, Pacific Tool had a go no/go set on the shelf for $250, shipped. A little bit spendy, but there ain't nothin' cheap about 20mm so I grabbed 'em.

Realistically, I probably have a gracious plenty of M55 projos as well. The trouble is, I'm an unrepentant hoarder (ask my wife). But, as the saying goes, "The guy with the biggest pile when he dies, wins", right?

What kind of scope holds up to 20mm recoil?

Not a thread hijack, just curiosity.

Good to know, I didn't think that Pacific offered anything but the reamers these days. $250 is a steal for a go/no go set for 20x102mm, too!



You might be surprised to know that the recoil impulse of 20x102mm isn't as bad as you might imagine. In fact, depending on the muzzle brake, weight, and so forth, the recoil might be on par with an 870 firing slugs. Basically any decent scope rated for .50BMG can be used, in my experience. Many optics companies have "heavy recoil" rated scopes that will work fine.

The real question is, what kind of scope both holds up fine to the recoil impulse and has a high enough magnification to make sense? 20x102mm can really reach out there.

Mine has a 5ft barrel and has a really long dwell and push using slow burning powder.

Still, gotta do something with 39,000 lb-ft of energy. Dead weight and recoil system to soak it up. Mine weighs 160lbs all up and has a 50lb spring in it.

I agree, any scope suitable for .50 BMG rifles will probably be fine for a 20mm. Just to throw out some names, Night Force and Leupold Mk. 4 scopes are top choices. I have always used Leupolds on my fifties and have been well satisfied. I have no experience with them, but other folks say they have gotten good results using Bushnell Tactical Elites, Sightrons and the SS Tactical scopes from SWFA. Some of the better European scopes would be excellent choices, too.
 
A shoulder fired 20mm will be 3 or 4 times heavier than the typical .50 BMG rifle and that extra weight does a lot to mitigate the recoil of the 20mm. People don’t realize that many times it isn’t recoil that kills scopes on big bore rifles, it’s the counter-recoil caused by the powder gas hitting the muzzle brake. That accelerates the gun violently forward so the scope is subjected to very high G-forces rearward and then forward over the span of just a few milliseconds. The scopes I mentioned above are built to stand up to those “push-pull” forces.

Hey, jestertoo, sounds like you are running an M61 Vulcan barrel. My barrel is 49" on an Anzio. Haven't had a chance to weigh the whole rig, but I'm guessing with the suppressor it'll go maybe  110-120 lbs.

Have to love those Vulcan barrels. I find the gain twist to be far more accurate than the PacNor 20mm barrels. Since realizing that, I've swapped all of them to NOS surplus Vulcan barrels,. Even though they're longer, the contour makes them about the same weight overall. It's nice.



If you're running an Anzio or other manual action, yes, unless it's a type with a recoilless system. Anything semi-auto tends to be a lot lighter thanks to the action spreading out the recoil impulse nicely. The lightest I've fired so far is a 20x110mm (USN) semi-auto that was about 50lbs. Sure kicked a bit more than a M82A1, but nowhere near as bad as I expected. Sadly, the barrel was only 36" long, which is a terrible waste of powder and far louder than it needed to be.

I don't think anzio shoots full power 580gr loads does he?

ETA is still want to shoot one :)

So far as I know, Mike loads his ammo to ordinary spec for the TP projectiles unless otherwise requested. I know he loaded a bunch in a past order for me to 590gr of WC872 behind a 1520gr TP bullet.

The Anzio handles full power loads. Here are a couple of videos that show military equivalent loads being shot. Near the end of the first video you can hear Mike Remo, the guy that runs Anzio, talking about the shot being a "max load of 600 grains of WC872 powder".

https://www.youtube.com/watch?v=NPmg7BHN75w

The second video shows the difference between a subsonic load (950 FPS) and a full power load (3300 fps). On the full power shot, watch for the bright flash at the top center of the screen when the projectile hits the big rock the shooter is aiming at. I guess that is the aluminum nose of the projo vaporizing on impact.

https://www.youtube.com/watch?v=lG1GrVbf5ho

That is correct. I have bought two different loadings from him, the full power ones and also a supply of some downloaded to 450gr of WC872 behind the same projectile.

I like Mike, he's a great guy, but his penchant for rounding important numbers in casual contexts bothers me greatly. Those are almost certainly 590gr loads of WC872, the factory spec for military training loads for 20x102mm rounds, not 600gr... which isn't in any loading that I'm aware of at all and is over a 1.6% increase in charge! He also calls the 1520gr (98.5 gram) TP projectiles "1500gr" in many instances, which leads to confusion as that's not the correct weight.

Actually, the military equivalent loads Mike did for me were 585 gr. of WC872.

Not to be too picky about it, but some military sources list loads as heavy as 605 gr. of WC872. TM 43 0001 27, (published 1994) lists Cartridge, Target Practice, M55A2 (1520 gr. projectile) as loaded with 605 gr. of WC872. A couple of other types (M56A3/M56A4, HEI and M53, AI) are specified in that same manual with a charge of 605 gr. of WC872 as well. A second, older US military reference I have shows the M55A2 TP cartridge loaded with “WC872, 600.0 Grains, Approx”. Of course, all of the above are with M52A3B electric primers and I don’t know if there is any difference in brisance between them and  the M36A2 percussion primers Mike uses.

All of that aside, anyone loading a cartridge they haven’t done before needs to drop the propellant charges by at least 10% below maximum  to be on the safe side. The same can be said any time different components are used and that’s especially true for big stuff like .50 BMG and 20mm.

Another thing that’s a wild card about reloading for the 20mm is the fact the 20x102mm isn’t exactly a high precision cartridge to begin with. According to the sources I have seen, specs for the components and accuracy on the M55A2 Target Practice round are as follows:

M103 cartridge case: 1575 ± 50.0 gr.
M55A2 projectile: 1521 ± 30.0 gr.  
Accuracy: average mean radius at 600 yds. < 15 inches

Now, take into consideration the only stuff us lowly civilian gun cranks have to work with is surplus that’s been sitting around for years and often has deteriorated due to less than ideal storage conditions. For me, all that adds up to two things; be extra careful with what you do and expect accuracy that may be ok, but not great, despite manufacturer's claims of “match grade” barrels, etc. After working your loads up, you may be able to put one through the barn door at 1000 yards but hitting the bull standing broadside inside ain’t gonna be a sure thing.  

GREAT information, thanks! I stand corrected.

Just one more thing and I’ll quit flogging this poor ol’ mule about powder charges, cartridge cases, projos, etc. The latest version of 20x102mm TP ammunition is the PGU-27/B round. That’s the one that has the projectile with the pointier ogive and a slight boat tail to improve aerodynamics. The projectiles for those cartridges are a little heavier than the M55A2 (1550 vs. 1520 gr.) and that extra weight should be taken into account when reloads using the PGU-27 tips are worked up. To keep life simple, I think 585-590 gr. of WC872 is a prudent maximum that shouldn’t be exceeded, irrespective of the tip used.  So, in the final analysis, I’m with pyotr_k on this one, despite what the military manuals say.

Sorry guys, I’m an engineer and tend to get wrapped up in details…

Another engineer! ME? I'm the same way about the details, it's why I feel comfortable designing and then machining my own large bore toys. The heat treat, well, I still farm that out to be 100% certain since I'm not too great at consistency, but everything else is genuinely entertaining to mess with. It's half the fun, really.

I say engineer, but I'm retired now. Does that make me an ex-engineer? I look at it the same way as someone who has been in the USMC; once a Marine, always a Marine. It gets in your blood; I still love to tinker and always will be fascinated by mechanical stuff so I reckon I'm still an engineer.  

I worked as a manufacturing engineer in the medium duty vehicle industry and was with the same company for my entire career. Wore a lot of different different hats during that time; designed tooling, set up manufacturing processes, did cost analysis, project management and technical writing. I figured when I retired, I'd get a lathe and mill and use my free time to build stuff. The trouble is I still have so many obligations that that "free time" seems to never materialize.

Free time has a way of being perpetually just around the bend! It's just the way of the world. You'll get there eventually, it's worth while to make time, even just an hour a month, to tinker with something you love in engineering.

My cannon I've been using 500gr of WC (evenly divides into 8lb jug of powder :).. OCD is kill oops forgot to wash my hands.

I got some of the hollow new PGU style projos. Gonna mix up some IM11 mix.

Wish ya'll were TX

I hear that! Not too many large bore DD owners overall, and I have yet to bump into more than one at shooting ranges... ever. It seems like talking about DDs online is the only way to bounce ideas around.

As far as the PGU style, did you buy those from CDVS or a different source? From what you said, it sounds like you're planning some fun. Going to use the normal aluminum nosecaps or do you have something special in mind? I prefer to use IM163 for the incendiary loads, but that's just my personal preference due to supplies I have on hand. Zirconium powder sure isn't cheap, but not only do I have a bunch already, I have plenty of both fine mesh magnalium and KClO4 from past pyrotechnic work. It ended up being cheaper than having to wait to pick up the barium nitrate from the next pyrotechnic meetup since my regular supplier said it can't be shipped anymore. The IM163 is just a 1:1 mix of IM23 and the super fine zirconium powder, I find it a little more powerful than IM11 or IM23 by itself, but all of the incendiary mixes work wonders.

How's the downrange accuracy with 500gr loads of WC872, is there any inconsistency? I wonder if it completely burns or if you're left with all sorts of debris in the barrel. There have been plenty of times when I would have preferred lighter loads for questionable berms, but all I have on hand are the 590gr charges.

I bought a few from CDVS to try out. Haven't bought any of the consumables for it yet tho, so open to suggestions for sure.

No accuracy tests at all, don't even have an optic mounted on it yet (whole thing got built warped due to welding and need rework it first). We've been just boresighting it on things <100 yards and putting holes in them :)

I'm also using swaged brass for BMG primers. I heard that messes with accuracy too. I haven't had any ignition problems or noticed crud in the barrel from the light charges.

ETA : sort of in order of things I need to do on my project:
1) weld up axle for M2 tripod to replace front leg, create mount for faux armor plate
2 )rebuild barrel mount to not be warped (bolted instead of welded).
3) sighting system

I built mine with a big muzzle brake. Works good, but mostly wanted it for the increased back blast :)

When you say consumables for loading the rounds, do you mean primers and powder, or the chemicals needed for IM11?

I hope you get things fixed up nicely, but it's good to hear you're able to enjoy it as-is. It would be lousy to be so close and unable to fire it at all.

As far as the swaged brass goes, I don't have any direct experience with that. All of my cases have inserts in the primer pockets instead of being swaged smaller. It was expensive in comparison to buying the brass already swaged to accept the .50BMG primers, but I feel that it was worth doing.

IM11

the swaged brass still needs something to keep the primer from seating deeper into the pocket. I had to figure that out the hard way, taking 2 hits every time to ignite.

IM11, as I'm betting you're already aware, is a 1:1 composition by weight of fine mesh barium nitrate and fine mesh magnesium/aluminum ("magnalium") alloy. My preferred supplier who does currently ship barium nitrate is Skylighter. They also sell magnalium, you would want the ~325 mesh type, but due to shipping regulations 51% of it by weight is large turnings and chips while 49% is the fine mesh powder you want. The easiest way to separate them out is using a window screen and sifting, the chips are left on the screen and the desired fine mesh powder falls right through. It's important to note that they may reject your order outright, as those two chemicals can easily be used to make illegal exploding fireworks. You may need to call or e-mail and explain your desired use if they reject the order, and even then you may be out of luck. Being that I've dealt with them for over a decade now, I don't tend to have any issues. You may need to purchase one item from them and the other from another supplier.

For making the composition, don't guess on weight, get an accurate scale and use it properly. For mixing, you need things like a dust mask, goggles, gloves, and no nearby sources of sparks or open flames if you value your life. Pyrotechnics isn't something to play with unless you always observe all safety precautions without exception. In the case of IM11, I wouldn't work with more than, at MOST, 50 grams at a time, so you'd need to weigh out 25g of each ingredient if you were to start off by making a huge batch.

Once you get one part of the barium nitrate and one part of the magnalium weighed out into separate containers, you need to finely powder the barium nitrate, as it clumps together over time due to moisture. The easiest way to do this on a budget and be safe about it is to use a laboratory-use ceramic mortar and pestle, and not use it for anything except oxidizers from then on. Once you get the barium nitrate powdered, confirm that the weight is still dead on 1:1 with the magnalium, and use a method of mixing called "diapering". The way to do this is to pour your measured powders in two separate piles on a sheet of paper, and then lift each corner of the paper in order, again and again. This lifts and gently mixes the composition together. Once mixed, you now have a dangerous, flammable, extremely hot-burning substance in front of you. Don't store it, don't screw with it, don't let it get wet - moisture wouldn't be ideal and may cause it to heat and ignite, or form even less stable chemicals. You then carefully scoop a small amount, using a plastic or wooden utensil or scoop, and put it into the empty cavity of the projectile you want to load as incendiary. Tamp gently into place until there is no movement and the cavity is filled just enough to allow the aluminum nosecap to seat fully. Make 100% certain the threads are absolutely and utterly devoid of any IM11 incendiary mix, or you run the risk of ignition while screwing the nosecap into place.

In a nutshell, that's how to load incendiary. The procedure is similar for other compositions, other calibers, and other types such as API, APIT, and so forth. The main difference is how things are loaded, as many smaller rounds are loaded from the base and then a plug is pressed and crimped into place.

Hopefully that helps! BE SAFE.

subscribing to the thread.   Finally tested my 20mm and would like to keep track of the thread.

That you on YouTube, posted tests 3 and 4?

That's a different fellow altogether. Shefron's rifle (sans recoil brake) may be seen on YouTube here.

Woof!

Yea it was a awesome day shooting the thing.   Next on the list is to build a bipod and build or adapt a muzzle break for the gun.
I have a build thread on Pirate4x4
http://www.pirate4x4.com/forum/outdoor-sports-recreation/1671362-20mm-rifle-build.html


Due to this thread I was finally able to find a No-go headspace for the rifle from Pacific tool and gauge.

http://www.pirate4x4.com/forum/outdoor-sports-recreation/1671362-20mm-rifle-build.html


Made hot. Man, I saw that a long time ago and since I don't keep up with P4x4 I missed most of the build, need for reloading stuff, etc. Sorry! Fun read.

Josh's uses electric primers.. Simple and sweet build

https://www.youtube.com/watch?v=mPGUutpARMQ