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81mm Mortar barrel/tube/build info (Page 13 of 15)
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The obturating ring style likely seals a lot better than the older rounds. The multiple groove design obturates with a venturi effect as the gases try to flow past the grooves. More back pressure will seal it better, but there will always be some blow by. An additional consideration, a lot of these rounds are reduced range for training. I do wonder if they were intentionally designed with extra windage to prevent them going as far. |
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Originally Posted By Sonoran_Tj: The obturating ring style likely seals a lot better than the older rounds. The multiple groove design obturates with a venturi effect as the gases try to flow past the grooves. More back pressure will seal it better, but there will always be some blow by. An additional consideration, a lot of these rounds are reduced range for training. I do wonder if they were intentionally designed with extra windage to prevent them going as far. Which ones? |
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Originally Posted By walkinginadangerzone: Which ones? Originally Posted By walkinginadangerzone: Originally Posted By Sonoran_Tj: The obturating ring style likely seals a lot better than the older rounds. The multiple groove design obturates with a venturi effect as the gases try to flow past the grooves. More back pressure will seal it better, but there will always be some blow by. An additional consideration, a lot of these rounds are reduced range for training. I do wonder if they were intentionally designed with extra windage to prevent them going as far. Which ones? The teardrop ones that a lot of got from Bowman, with the holes in the front and back. Those holes are original, not a demill. They're made to launch really short range with a full power charge for training. I know the speed holes are by design, I'm just surmising that the obturator may be designed for a similar purpose. Would be neat to get ahold of an HE body of the same design and measure the dimensions against the training round. |
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Originally Posted By Sonoran_Tj: The teardrop ones that a lot of got from Bowman, with the holes in the front and back. Those holes are original, not a demill. They're made to launch really short range with a full power charge for training. I know the speed holes are by design, I'm just surmising that the obturator may be designed for a similar purpose. Would be neat to get ahold of an HE body of the same design and measure the dimensions against the training round. Originally Posted By Sonoran_Tj: Originally Posted By walkinginadangerzone: Originally Posted By Sonoran_Tj: The obturating ring style likely seals a lot better than the older rounds. The multiple groove design obturates with a venturi effect as the gases try to flow past the grooves. More back pressure will seal it better, but there will always be some blow by. An additional consideration, a lot of these rounds are reduced range for training. I do wonder if they were intentionally designed with extra windage to prevent them going as far. Which ones? The teardrop ones that a lot of got from Bowman, with the holes in the front and back. Those holes are original, not a demill. They're made to launch really short range with a full power charge for training. I know the speed holes are by design, I'm just surmising that the obturator may be designed for a similar purpose. Would be neat to get ahold of an HE body of the same design and measure the dimensions against the training round. The bowman teardrop practice rounds with the huge holes are very similar to the solid HE rounds. Last time I checked, the only difference between both was the weight, with the practice round being lighter because of the missing mass from the cut out disks. |
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Got some data yesterday. Small sample, but it's a start. I made a 3D printed Gunners Quadrant to use and it worked well. It was within a degree of my phone inclinometer. Attached File First, my 3D printed rounds weigh 4lb 2oz with fins. The diameter at the widest is 3.120" The M880 weighs 7lb 5oz with a dummy tip and has an OD of 3.173" I shot a pair 3D printed rounds 3 times with 50gr of TiteGroup and Bullseye. Twice at 70° and once at 60° Bullseye got an average of 50% more distance. Still only maxed out at 55yd with 70° I shot my M880 with 50gr of TiteGroup at 70° and got 115yds. It went way up... and completely buried itself. I almost lost it. Attached File After digging it out I shot it again. I was out of TiteGroup charges so I used 50gr of Bullseye, but I removed the obturating ring. This got me 74 yards. Given the increase from using bullseye, I'd say the obturating ring makes a big difference. |
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Originally Posted By MHIDPA: Lol, I have considered air tags Originally Posted By MHIDPA: Originally Posted By backbencher: With you guys that have handy forests & going past 200 yds, have you considered a fishing line on a spool?  Lol, I have considered air tags Air Tags could give you apogee & location. Actually, if you really want to go down a rabbit hole, the amateur rocket folks have tiny GPS systems that broadcast & record all sorts of info, and they're putting serious Gs on those systems, they work pretty well. Might not work at max charges but for ordinary practice stuff should run alright. Might get expensive if you're wanting to fire a lot of rounds before recovery. |
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Originally Posted By MHIDPA: Lol, I have considered air tags Originally Posted By MHIDPA: Originally Posted By backbencher: With you guys that have handy forests & going past 200 yds, have you considered a fishing line on a spool? Lol, I have considered air tags I tried the samsung tags but considering the rounds are made of thick steel, they block off the signal. So that plan didn't work.
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The airtags and whatnot sound cool but the signal would probably be mostly blocked. The model rocket computers aren't a bad idea and could get you some cool data if you had a lot of time to do that kinda stuff. Would help to pot them in epoxy and align all the parts with recoil so nothing breaks. But borrowing a much simpler tech from model rocketry, you could put a audible beeping tag in a shell. Then you just need to get close enough and your ears will guide you. Or rub them in bacon and bring a dog. |
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Originally Posted By MHIDPA: Got some data yesterday. Small sample, but it's a start. I made a 3D printed Gunners Quadrant to use and it worked well. It was within a degree of my phone inclinometer. https://www.ar15.com/media/mediaFiles/60540/1000004430_jpg-3512244.JPG First, my 3D printed rounds weigh 4lb 2oz with fins. The diameter at the widest is 3.120" The M880 weighs 7lb 5oz with a dummy tip and has an OD of 3.173" I shot a pair 3D printed rounds 3 times with 50gr of TiteGroup and Bullseye. Twice at 70° and once at 60° Bullseye got an average of 50% more distance. Still only maxed out at 55yd with 70° I shot my M880 with 50gr of TiteGroup at 70° and got 115yds. It went way up... and completely buried itself. I almost lost it. https://www.ar15.com/media/mediaFiles/60540/1000004431_jpg-3512242.JPG After digging it out I shot it again. I was out of TiteGroup charges so I used 50gr of Bullseye, but I removed the obturating ring. This got me 74 yards. Given the increase from using bullseye, I'd say the obturating ring makes a big difference. Nice quadrant set up. We had a massive rain the week before I went out to shoot mine. The dirt here is usually very compact and dry, but since we got a lot of rain, the dirt was still soft under neath, they buried themselves about 1 1/2 feet. |
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I'm pretty excited, I got the heaviest 10rd box of ammo ever today. Picked up 10 M880 Training rounds for a great price. Attached File Attached File I've got to make fuse/tips for them and weld some holes, but with 13 total, I'm not chasing 3D printing for a while. |
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Originally Posted By MHIDPA: I'm pretty excited, I got the heaviest 10rd box of ammo ever today. Picked up 10 M880 Training rounds for a great price. https://www.ar15.com/media/mediaFiles/60540/1000004570_jpg-3532201.JPGhttps://www.ar15.com/media/mediaFiles/60540/1000004571_jpg-3532202.JPG I've got to make fuse/tips for them and weld some holes, but with 13 total, I'm not chasing 3D printing for a while. Nice, I got 6 of the M879A1 practice rounds. Wish I would've gotten more but I already had 6 solid M374 shells which I might try and convert to the M879 practice rounds. Will eventually make the fin adapters. 
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Originally Posted By walkinginadangerzone: Nice, I got 6 of the M879A1 practice rounds. Wish I would've gotten more but I already had 6 solid M374 shells which I might try and convert to the M879 practice rounds. Will eventually make the fin adapters. https://i.imgur.com/v4NUNzN.jpeg Nice, are you planning on getting obturating rings? |
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Originally Posted By MHIDPA: Nice, are you planning on getting obturating rings? Originally Posted By MHIDPA: Originally Posted By walkinginadangerzone: Nice, I got 6 of the M879A1 practice rounds. Wish I would've gotten more but I already had 6 solid M374 shells which I might try and convert to the M879 practice rounds. Will eventually make the fin adapters. https://i.imgur.com/v4NUNzN.jpeg Nice, are you planning on getting obturating rings? Yep, will eventually order them from destructivedevices. |
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Just thought I'd share how the M879A1 practice rounds work. They have a "barrel" fixed to the end of the fuze so upon impact, the gasses travel down the barrel and through the exit holes on the bottom of the body, blowing out the metal plugs. The barrel has an O ring on the end to minimize bleed off inside the mortar body. The barrels are hefty, probably to help the practice round mimic the weight of the real HE rounds.   
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That's cool. I don't know much about the M880 except there are 2 sets of plugs at the bottom. One set is removable to adjust range and stimulate increment charges. The other are called dud plugs. My understanding is that the dud plugs pop out when the spotting fuse detonates. This gives a visual indication upon recovery to prevent dealing with UXO. Also, there seems to be 2 versions of the M880, with and without dud plugs. The M880 also has vents on the top. As far as I can tell, it doesn't have a "barrel' Attached File |
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Originally Posted By MHIDPA: Back to working on fuses? Originally Posted By MHIDPA: Originally Posted By Ben: Does anyone have the M43A1 HE round TDP? Back to working on fuses? Actually, have the ability to make mass produced complete copies of originals at this point. But not going to do that for sales until either a)extensive testing, or b) I have the original TDPs to make sure everything is on spec. |
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After reading up on the M880, I got to thinking about what I could use as a dud plug. I pulled one of the ones I had out (it was stuck pretty good). It is just a "Christmas tree clip" like you would use on car bodywork or cheap furniture. I bet it would also work on your M879s too. Attached File |
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Originally Posted By MHIDPA: After reading up on the M880, I got to thinking about what I could use as a dud plug. I pulled one of the ones I had out (it was stuck pretty good). It is just a "Christmas tree clip" like you would use on car bodywork or cheap furniture. I bet it would also work on your M879s too. https://www.ar15.com/media/mediaFiles/60540/1000004595_jpg-3535595.JPG Yeah, any plugs should work as long as they fit somewhat tight. Will eventually 3d print them if I ever get a 3d printer. Lol |
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Originally Posted By Ben: Actually, have the ability to make mass produced complete copies of originals at this point. But not going to do that for sales until either a)extensive testing, or b) I have the original TDPs to make sure everything is on spec. Originally Posted By Ben: Originally Posted By MHIDPA: Originally Posted By Ben: Does anyone have the M43A1 HE round TDP? Back to working on fuses? Actually, have the ability to make mass produced complete copies of originals at this point. But not going to do that for sales until either a)extensive testing, or b) I have the original TDPs to make sure everything is on spec. Which fuzes? |
I'm planning on trying to do that later this summer to see how the 81mm practice shells work. The only setback is trying to find a tube sleeve that the shotshell goes in and that is the same diameter as the fuze opening and that can also fit a 20guage shotshell. The OG sleeves fit the Activ plasitc hulls, but won't allow the plastics hulls with brass to seat all the way in. 
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Originally Posted By MHIDPA: I have a lathe, boring it out wouldn't be a problem, when you want to try give me a shout.. Did you have to do anything special to get your tube to screw out? Mine won't budge. IM Sent, But what tube? This one? 
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So what I've found, they are in there with some sort of epoxy. I tried using heat to the fuze head, and it didn't budge, tried 3 different ones. Gouged the shit out of them with knipex pliers lol. On two other ones, they came right off with some brute force, tubes were pretty gouged, but still usable. The epoxy is like a white paste looking thing, almost like liquid nail. |
Regarding reloading of the M751 practice fuze. I'm not sure it would be possible to do it easily.  1. External safety pin 2. Internal setback slider pin 3. Firing pin holder 4. Firing pin 5. Ball bearing, act as the firing pin striker 6. Primer in 20 Gauge shotshell blank 7. Blank 20 Gauge shotshell spotter cartridge The access holes are staked so you can't just unscrew a cap or something and access the parts inside. I found it's easy to just drill out the staking and then you can disassemble the fuze parts but you won't be able to hold them in as they are spring loaded. You might be able to epoxy the opening shut once you re-arm the fuze though. For example, the ball bearings (5) that act like the firing pin face, not sure if you can shove them back into the channel but pressing on them through the firing pin channel (6). For resetting the setback slider pin (2) you can drill out the bottom staking, and thread on a little set screw to hold it in place and make it easier to re-arm. From there, remove the spring, set back pin, and then with a paperclip or equivalent push in the firing pin holder (3) while at the same time pressing in the firing pin (4). The firing pin holder (3) staking doesn't have to be drilled out as you can just push it back into place with a small screwdriver or paperclip type wire. And something to consider, there might be a reliability issue with the fuze at charge 0. From the PDF linked below. During PQT of the M879 cartridge (M751 Fuze), a ballistic reliability of 27.2% for charge 0 was demonstrated. Download link to PDF on the practice fuze |
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Edit: Internals should be the same. I think it's an earlier version of the practice fuzes. I found references to it being used on the 60mm M766 SRTR projos, similar to your 81mm rounds.  Also found this, so it was used on the 60mm practice rounds before they upgraded to the current ones. This specification covers the manufacture of metal parts and pyrotechnic signature cartridges for one type of point detonating practice fuze designated as Fuze, Point Detonating, Practice, M779, for use on Cartridge, 60MM, Practice, Short Range, M766. |
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I have also started working on my own "TRUMP" style fuse. I went with a wider angle to hopefully reduce penetration. Attached File |
| Just this one for now. A buddy blasted and powder coated it. I want to make sure the thickness of paint doesn't affect firing before I do them all. The fuze well threads are 1 1/2-12 as far as I can tell that is standard across US mortars. I got a tap off eBay to chase threads in the rusty M880 bodies. |
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Regarding, re-arming the M751 style fuzes. Tools.  First you have to drill out the staking on the bottom of the fuze in order to pull out the spring and the setback pin. You should be able to thread the hole and add a set screw to keep it in place for multiple uses. Just need to trim the Setback spring.  Then with the fuze opening facing up, place one of the Ball bearings a/k/a BBs back in the hole, with the firing pin in the armed position, the BB should just fall back into the hole.  Slightly use the firing pin by pressing it in slightly to move the BB forward so it falls back into the BB channel. Make sure to orient the fuze with the setback pin hole facing up, otherwise the BB spring will just fall into the firing pin channel and you'll have a hard time putting them back in. Once you see the BB slightly fall in, use a small glasses screwdriver to push the BB down into the channel then press the firing pin all the way in, this will block the BB channel and keep it inside while you prep the other one. You can barely make out the BB at the bottom.  Now do the same with the 2nd BB, but don't let go of the firing pin completely as you'll have to start over. While still holding the firing pin firmly, place the 2nd BB in the hole, slightly release the firing pin, while at the same time pushing the 2nd BB in to the firing pin channel, eventually you'll reach the BB channel and then you can use the screw driver to move it down into the BB channel, at this point, re-press the firing pin all the way to lock them in place. Now while still holding the firing pin in place, put the firing pin holder and spring into their holes, and with the small screw driver push it all the way in, you can now release pressure from the firing pin as the firing pin holder will keep the firing pin in place. Now just re-insert the setback slider pin and make sure it goes through the firing pin holder hole, now you can press the firing pin down as this will release the firing pin holder to sit against the setback slider pin. Now just insert the setback slider pin spring, and set screw and lock it in place. If your fuze still has the wind screen plastic, don't remove it as it will catch the firing pin holder once the fuze arms. You can make a little hole through the windscreen in order to poke the screw driver through just enough to reset the firing pin holder. If your fuze didn't come with the plastic windscreen, just place a strip of duck tape around the fuze and firing pin holder hole. |
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Went out to the OH DD shoot this weekend. I got to try out my metal sight mount adapter and it worked great. I compared the settings to my phone angle finder app and it was dead nuts Attached File I made a home brew version of the TRUMP fuse and it worked great. One big lesson learned for anyone who might be using training rounds. The M880 uses a firing pin inside a plug in the fins. I only had 2 plugs so in order to use my fins I made threaded bushings that replaced them. This worked great BUT my bushings were shorter than the length of the brass on my hulls. So when the round fired, the unsupported brass on the inside of the fin flared out and locked it in place. I had to remove the bushing with the fired hull. Crimp the brass with pliers and then punch out the hull. I will be remaking the bushings. Lol |
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My 60mm PD tips worked really well too. They are fully 3d printed except for a single screw. I am going to make a version for m49 rounds since sherluk has a bunch available. They'd probably be easy to adapt to any round with a screw in fuze. I just had a box of them and screw a new one in before dropping the round. For anyone that prints, it's just the 40mm PD projectile file from launcherpope.com, that is spliced on a thick set of threads for a base. I print the first 20 layers or so 100% infill and that protects the projectile body from the blast. Surprisingly, I don't think I broke a single printed round or fin. |
81mm Mortar barrel/tube/build info (Page 13 of 15)
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