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Built an AR10 in 280 British (Page 1 of 2)
Posted: 12/18/2025 10:08:39 PM EDT
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Thought I'd share this recent project I finally finished. AR10 chambered in the original late '40s 280 British. I like to put together ARs in non standard calibers and this one was always one I was interested in trying. Did it as a sort of "what if" thing where the 280 had been adopted by NATO instead of the 7.62. I surmise that had that been the case the AR would not exist in a small and large frame as it is now but something in between to accommodate the size of the 280 Brit. Just an interesting thought experiment. Took most of two years to scrape it all together. Reamer and gauges came from JGS and took about 9 months. Dies came from Redding and took about 4 months. Barrel was an X-caliber carbon 7mm blank and took about 4 months between waiting on the barrel and then the smith to cut the chamber. Once I got the ammo issues sorted, which took some time, it now runs with no issues and is quite pleasant to shoot. Have run about 500rds through it at this point. Accuracy has also turned out to be very good as well and will shoot sub MOA and I've run it out to 600yds so far.  Worked with JGS to design the reamer. My first trip through the custom reamer design process. Kinda cool looking. 
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Originally Posted By ALASKANFIRE: How close is that to a 7BR? 7mmBR has an about 39 grains of water capacity and my converted brass for the 280 Brit is about 41.5. Also the BR has a 38mm case and the Brit is 43mm. BR will run a 140g bullet about 2-300fps slower in similar length barrel both running the same pill. These were my numbers with a 139g SST 
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Originally Posted By ALASKANFIRE: Nice. I appreciate all you weirdos. Lol this is probably one of my least weird caliber oddballs. My weirdest one I'm trying to get working is an AR chambered in 32acp using a Beretta mag but that one will deserve its own thread eventually. |
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Originally Posted By Sinister: Where or how do you source brass for it? I gots to know. Very cool. The original 280 British had a proprietary case head. Later they enlarged it slightly to match the size of the US T65 cartridge it was competing against that would end up as the 7.62 NATO. This was done to get closer to the performance of the T65 by increasing case volume. When they did this it was called the 280/30 British which is a bit misleading as it's a 7mm round. All that being said it allows conversion of 308 brass to fabricate the 280 Brit so in reality what I'm making is technically the later 280/30 version. The is the version that was briefly adopted unilaterally by the British as the 7mm Mk1 Z with a 140g bullet along with the EM-2 rifle. This pic is early in the trials and tribulations of working out the conversion. Eventually I ended up converting Nosler 243 brass which ended up with thinner necks that I don't have to turn down. The 308 brass ended up with bad donuts from forming and when I ran the mandrel through the neck the donut got pushed to the outside and caused case necks to jam in the chamber hence having to be turned. The 243 eliminated the need to turn and life is good. Pains of having to reverse engineer what was only really ever a prototype.  |
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Originally Posted By stiglitz_actual: Heck of a post for only your 8th  Love to see these kind of projects, thanks for sharing! Thanks. Been wanting to share some of my work for several years now as I'm always working on the next thing. I just don't play on forums much anymore due to work travel, reloading, occasional pistol matches and honestly tinkering with things like this instead of computer time. Recent knee surgery and the usual work travel holiday slow down has got me on the forums again which I haven't done since my days playing with car stuff. Also am an avid collector of Berettas with 44 currently in hand going back to the original model 1915 but I'll mess with that over in the handgun forum. |
I think a 10 shot group at 100yd indoor range but might be 20. Did some of both. This was a 120g Nosler boat tail which is what I ended up settling on. The 280 Brit used 130g and 140g but never used a 120g pill but with the results I got I stuck with it and figured if I'm doing a modern take I'll just chock it up to advancements in bullet design and run with it. Velocities with the 120g are about 2600fps.
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Awesome! I often lament the rejection of this round in favor of 308. I believe you're correct in that there would most likely be only 1 size AR and 5.56 would possibly have never existed. Closest I came to what you have is a 6.5G....which you have now made to be quite lame. |
You are a gentleman and a scholar. I believe that if we had adopted this combination we would still be using it. Now you need to find a BRN-10 to rebarrel for a proper 1950’s prototype |
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Originally Posted By TonyRumore: His reamer is calling out .284" The lead/freebore in front of the case mouth is .285". Tony Thanks Tony for picking that one up for me. Was out of pocket for a rifle class and couldn't jump on for a few days. Yes standard .284 pills. I won't sit here and say I completely understand all the in and out of the reamer print. Have a pretty good idea on the majority of it but the rest I wisely left to the professional folks. The main thing I did was specify the jump. They design all their reamers to have the bullet engage the rifling but I wanted some room to work with different bullets and seating depth so I could find the best combo since it was all an unknown for the most part. |
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That's awesome, OP. And looking forward to your AR-32, please post it in this forum. You're aware of the Philippine "AR-32"? @HenryKnoxFineBooks |
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 Did You Know Italy Made a .32 Colt SP1 AR15 Clone?(Armi Jager AP74 7.65mm Rifle Review) |
We are living in a Tom Clancy novel
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Originally Posted By backbencher: That's awesome, OP. And looking forward to your AR-32, please post it in this forum. You're aware of the Philippine "AR-32"? @HenryKnoxFineBooks Have been aware of the Army Jeager for several years now and would love to get my hands on one without breaking the bank. As for the Philippine 32 I'll definitely look into that for sure and thanks for the link. I've said for years now that in the firearms world and especially the cartridge side of things not much is new. I've not concocted to AR10 nor the 280 Brit just put the two together. |
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Originally Posted By tsymonds: Have been aware of the Army Jeager for several years now and would love to get my hands on one without breaking the bank. As for the Philippine 32 I'll definitely look into that for sure and thanks for the link. I've said for years now that in the firearms world and especially the cartridge side of things not much is new. I've not concocted to AR10 nor the 280 Brit just put the two together. Originally Posted By tsymonds: Originally Posted By backbencher: That's awesome, OP. And looking forward to your AR-32, please post it in this forum. You're aware of the Philippine "AR-32"? @HenryKnoxFineBooks Have been aware of the Army Jeager for several years now and would love to get my hands on one without breaking the bank. As for the Philippine 32 I'll definitely look into that for sure and thanks for the link. I've said for years now that in the firearms world and especially the cartridge side of things not much is new. I've not concocted to AR10 nor the 280 Brit just put the two together. I may be thinking of the Italian AR-32, maybe the Philippines didn't produce one. |
I sell firearms produced by the finest child labor in the world, be it Filipino, Muslim, Mormon, Arizonan, or Texan.
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Originally Posted By tsymonds: Lol this is probably one of my least weird caliber oddballs. My weirdest one I'm trying to get working is an AR chambered in 32acp using a Beretta mag but that one will deserve its own thread eventually. Originally Posted By tsymonds: Originally Posted By ALASKANFIRE: Nice. I appreciate all you weirdos. Lol this is probably one of my least weird caliber oddballs. My weirdest one I'm trying to get working is an AR chambered in 32acp using a Beretta mag but that one will deserve its own thread eventually. How about .276 Enfield and .276 Pedersen two cartridges which might’ve become standard but for the coming of two world wars. .276 Enfield has a projectile weight of 165 gr (10.69 g) and a muzzle velocity of 2,785 ft/s (849 m/s). Would need a .30-06 length action. .276 Pedersen produced velocities of around 2,400 feet per second (730 m/s) with 140 or 150 grain (9.1 or 9.7 g) projectiles. |
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Originally Posted By Capta: How about .276 Enfield and .276 Pedersen two cartridges which might’ve become standard but for the coming of two world wars. .276 Enfield has a projectile weight of 165 gr (10.69 g) and a muzzle velocity of 2,785 ft/s (849 m/s). Would need a .30-06 length action. .276 Pedersen produced velocities of around 2,400 feet per second (730 m/s) with 140 or 150 grain (9.1 or 9.7 g) projectiles. Either would be a fun project but as you said the length of the cartridge would be an issue. Those would be another one for a magnum frame AR but can't get the parts. |
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Originally Posted By tsymonds: Either would be a fun project but as you said the length of the cartridge would be an issue. Those would be another one for a magnum frame AR but can't get the parts. Originally Posted By tsymonds: Originally Posted By Capta: How about .276 Enfield and .276 Pedersen two cartridges which might’ve become standard but for the coming of two world wars. .276 Enfield has a projectile weight of 165 gr (10.69 g) and a muzzle velocity of 2,785 ft/s (849 m/s). Would need a .30-06 length action. .276 Pedersen produced velocities of around 2,400 feet per second (730 m/s) with 140 or 150 grain (9.1 or 9.7 g) projectiles. Either would be a fun project but as you said the length of the cartridge would be an issue. Those would be another one for a magnum frame AR but can't get the parts. .276 Pedersen could be done in an AR10. (51.4mm case) Personally I think the 276 Enfield is more interesting. It was ahead of the tech of the time and limited by cordite propellant. It has a decently long neck and, if using a 30-06 length magazine, looks like you could load some long bullets in the case. 59.7mm case. Of course, ballistically it would be just an oddball and expensive .270win.  30-06 length action available here. |
No man knows the day and the hour, but if the time comes, better to stand with the righteous than piss your pants like a coward.
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Originally Posted By Capta: .276 Pedersen could be done in an AR10. (51.4mm case) Personally I think the 276 Enfield is more interesting. It was ahead of the tech of the time and limited by cordite propellant. It has a decently long neck and, if using a 30-06 length magazine, looks like you could load some long bullets in the case. 59.7mm case. Of course, ballistically it would be just an oddball and expensive .270win. http://photos.imageevent.com/badgerdog/cgnmilsurpknowledgebase/britishpedersensemiauto/cartridges.jpg 30-06 length action available here. Guess I need to brush up a bit on the oddballs as I wasn't aware of the case length of the Pederson. Agree on the 276 Enfield as far as potential but a bit much work to conjure up what you accurately describe as expensive 270 Win. I was also unaware that BCA had made some magnum length stuff available and really appreciate the link. That'll warrant a call to them to see what I can come up with to try and stuff 7.5 Swiss into a gas gun. Might just end up as the equivalent to 308+P I suppose but it definitely is a very good round from my K11 and I already have dies and brass. |
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Originally Posted By tsymonds: Guess I need to brush up a bit on the oddballs as I wasn't aware of the case length of the Pederson. Agree on the 276 Enfield as far as potential but a bit much work to conjure up what you accurately describe as expensive 270 Win. I was also unaware that BCA had made some magnum length stuff available and really appreciate the link. That'll warrant a call to them to see what I can come up with to try and stuff 7.5 Swiss into a gas gun. Might just end up as the equivalent to 308+P I suppose but it definitely is a very good round from my K11 and I already have dies and brass. 👍👍 |
No man knows the day and the hour, but if the time comes, better to stand with the righteous than piss your pants like a coward.
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Changed the optic from the 1-8 LPVO to a 5-25 that I can dial and work out a dope for the 280. Was able to run out to 800yds in 100yd increments. 120g Nosler ballistic tip boat tail. Here's the distances and dial ups. 200 1.5 300 4.0 400 8.25 500 12.25 600 17.5 700 22.5 800 31.25 Here's my 308 AR same 16" barrel length and running 168g SMK for reference. 200 1.5 300 4.0 400 7.25 500 11.0 600 15.25 700 20.0 800 25.75 And my 20" 260 AR running 130g ELD-m 200 1.25 300 3.75 400 7.0 500 10.5 600 14.5 700 19.0 800 24.5 And my 14.5" 223 AR running 55g FMJBT 200 1.25 300 3.75 400 7.0 500 11.5 600 17.0 700 24.0 800 32.5 Can see that at least as far as drop goes it's closer to my 223 than the others but with over double the bullet weight. A bit more drop to 400 but after that the better BC and weight starts to come into play vs the 55g pill. |
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Definitely a cool thread/project. The talk of the 276 pederson also puts me in mind of the 6mm Lee Navy . One of my favorite pet cartridges has long been the 7-30 Waters . Reconstructing it as a rimless , higher pressure cartridge by turning the rim off of 375 Winchester brass would just about get me there . Then Id just have to decide .257 , .264 , .277 , or .284 . I lean towards .277 or .284 for more bore area as I prefer a shorter bbl . Case capacity would be 45ish gr . |
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Originally Posted By 9D1Alpha: Definitely a cool thread/project. The talk of the 276 pederson also puts me in mind of the 6mm Lee Navy . One of my favorite pet cartridges has long been the 7-30 Waters . Reconstructing it as a rimless , higher pressure cartridge by turning the rim off of 375 Winchester brass would just about get me there . Then Id just have to decide .257 , .264 , .277 , or .284 . I lean towards .277 or .284 for more bore area as I prefer a shorter bbl . Case capacity would be 45ish gr . Interesting possibilities there |
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Originally Posted By 9D1Alpha: Definitely a cool thread/project. The talk of the 276 pederson also puts me in mind of the 6mm Lee Navy . One of my favorite pet cartridges has long been the 7-30 Waters . Reconstructing it as a rimless , higher pressure cartridge by turning the rim off of 375 Winchester brass would just about get me there . Then Id just have to decide .257 , .264 , .277 , or .284 . I lean towards .277 or .284 for more bore area as I prefer a shorter bbl . Case capacity would be 45ish gr . At their first meeting, 6mm Lee smoked 7mm Mauser - but that's more b/c the Marines had the high ground and the Spanish had no cover. The Marines probably would have won that fight w/ .45-70, but a modern smokeless HV cartridge helped. |
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Originally Posted By tsymonds: Interesting possibilities there Originally Posted By tsymonds: Originally Posted By 9D1Alpha: Definitely a cool thread/project. The talk of the 276 pederson also puts me in mind of the 6mm Lee Navy . One of my favorite pet cartridges has long been the 7-30 Waters . Reconstructing it as a rimless , higher pressure cartridge by turning the rim off of 375 Winchester brass would just about get me there . Then Id just have to decide .257 , .264 , .277 , or .284 . I lean towards .277 or .284 for more bore area as I prefer a shorter bbl . Case capacity would be 45ish gr . Interesting possibilities there You're project provides inspiration. I also lean towards .277 for expediency . A reamed out 6.8 spc bbl perhaps . I got excited when the SFAR came out and started going down the path of what I'd need . I now have two POF firing pins with some mods . I try to procrastinate to slow down the insanity. * probly easier to get the 7-30 Waters bbl spun up and use the 7-30 Waters dies |
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Originally Posted By backbencher: At their first meeting, 6mm Lee smoked 7mm Mauser - but that's more b/c the Marines had the high ground and the Spanish had no cover. The Marines probably would have won that fight w/ .45-70, but a modern smokeless HV cartridge helped. Originally Posted By backbencher: Originally Posted By 9D1Alpha: Definitely a cool thread/project. The talk of the 276 pederson also puts me in mind of the 6mm Lee Navy . One of my favorite pet cartridges has long been the 7-30 Waters . Reconstructing it as a rimless , higher pressure cartridge by turning the rim off of 375 Winchester brass would just about get me there . Then Id just have to decide .257 , .264 , .277 , or .284 . I lean towards .277 or .284 for more bore area as I prefer a shorter bbl . Case capacity would be 45ish gr . At their first meeting, 6mm Lee smoked 7mm Mauser - but that's more b/c the Marines had the high ground and the Spanish had no cover. The Marines probably would have won that fight w/ .45-70, but a modern smokeless HV cartridge helped. A cartridge ahead of its time . And God bless the US Marines . That 7mm Mauser though ... |
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tsymonds, Very nice build done right. Have wondered what the 43mm case of the SPC would be like loaded long with ELDs and to the pressure that a .308 AR can tolerate. But, the combination does not like .308 magazines and the ELDs wont work in the 7.62x45cz magazines... Since I started with the magazines, for me that killed the consideration of the 6.8 spec 3. If it were practical, I would love to compare what 6.8 could deliver, with your .280Brit. db edited: should just build a bolt gun. But then it would only be a more expensive under achieving 6.5 creed. |
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Originally Posted By tsymonds: The original 280 British had a proprietary case head. Later they enlarged it slightly to match the size of the US T65 cartridge it was competing against that would end up as the 7.62 NATO. This was done to get closer to the performance of the T65 by increasing case volume. When they did this it was called the 280/30 British which is a bit misleading as it's a 7mm round. All that being said it allows conversion of 308 brass to fabricate the 280 Brit so in reality what I'm making is technically the later 280/30 version. The is the version that was briefly adopted unilaterally by the British as the 7mm Mk1 Z with a 140g bullet along with the EM-2 rifle. This pic is early in the trials and tribulations of working out the conversion. Eventually I ended up converting Nosler 243 brass which ended up with thinner necks that I don't have to turn down. The 308 brass ended up with bad donuts from forming and when I ran the mandrel through the neck the donut got pushed to the outside and caused case necks to jam in the chamber hence having to be turned. The 243 eliminated the need to turn and life is good. Pains of having to reverse engineer what was only really ever a prototype. https://imagizer.imageshack.com/img924/2302/IRNxmg.jpg The change in head size was due to existing 8mm Mauser production tooling being available at Radway Green. 8mm Mauser was being produced there for the BESA Tank Machine Gun. If you look closely at the case drawing, the dimensions are for the Mauser rim, not the wider extraction groove used on the FAT1E3 case. The original head used was the same as .276 Pedersen, which was also briefly produced in Britain. |
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Originally Posted By lysanderxiii: The change in head size was due to existing 8mm Mauser production tooling being available at Radway Green. 8mm Mauser was being produced there for the BESA Tank Machine Gun. If you look closely at the case drawing, the dimensions are for the Mauser rim, not the wider extraction groove used on the FAT1E3 case. The original head used was the same as .276 Pedersen, which was also briefly produced in Britain. Very interesting and was not aware of that. I recall the BESA vaguely from some prior reading but had never tied anything together regarding either case sizes but makes perfect sense. Appreciate the info. |
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Originally Posted By wganz: there is always the 7x33 Sako for a quick & easy project for you. Linkage Oof another rabbit hole. Gotta dig into that one a bit. Looks sufficiently odd for me to stick in an AR. |
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Originally Posted By wganz: there is always the 7x33 Sako for a quick & easy project for you. Linkage 350 legend brass would make quick work of it . 6.8 bullets might better serve it going into the lighter spectrum. You could run them quite a bit harder than the original operating pressure. * 23gr capacity. Looks closer to 300blk than 7.62x25tok . |
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Originally Posted By 9D1Alpha: 350 legend brass would make quick work of it . 6.8 bullets might better serve it going into the lighter spectrum. You could run them quite a bit harder than the original operating pressure. * 23gr capacity. Looks closer to 300blk than 7.62x25tok . Already have a Tokarev AR so that's covered. The 7x33 has my interest and being able to use relatively easy to find brass would be a definite bonus. |
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Originally Posted By tsymonds: Already have a Tokarev AR so that's covered. The 7x33 has my interest and being able to use relatively easy to find brass would be a definite bonus. Originally Posted By tsymonds: Originally Posted By 9D1Alpha: 350 legend brass would make quick work of it . 6.8 bullets might better serve it going into the lighter spectrum. You could run them quite a bit harder than the original operating pressure. * 23gr capacity. Looks closer to 300blk than 7.62x25tok . Already have a Tokarev AR so that's covered. The 7x33 has my interest and being able to use relatively easy to find brass would be a definite bonus. Looking forward to the possible next project  * a hypothetical historical I'm looking at is the would have been 7x33 based off the 7.92x33 . Id much considered the 25-204 ruger or rimless 7-30waters , but the cast off parts of the sfar leave a heavy duty bolt and bbl extension. |
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Originally Posted By tsymonds: Here's a pic for comparison. 308, 280 Brit, 223 and 221 Fireball https://imagizer.imageshack.com/img922/343/51sSJd.jpg Cool project. |
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Originally Posted By SteelonSteel: Very cool project. Now I want a L1 service rifle in .280/30.  |
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Built an AR10 in 280 British (Page 1 of 2)
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