.303 British to .410 Shotshell Conversion

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So, I’m a little pumped about being able to reload my own .410 shot-shells without a shot-shell reloading press so I head out to the not-so-local skeet range to do some hull scavenging.  I get there a day after a corporate shoot expecting to find at least a small assortment of .410 hulls –  and not a single one was to be found anywhere.  Disappointed in my efforts to locate some .410 hulls I resorted to the internet and searched for once-fired .410 hulls.  Interestingly, during my search I found some periodic comments and rough tutorials on the internet on how to convert .303 British brass to .410 shot-shell hulls.  Yes, I know – you can purchase brass hulls from MagTech – but if I can make my own .410 hulls out of some once-fired .303 British brass, now that really sparks my interest.  You see – anytime I can make something myself cheaper than buying it at retail prices I get interested, even if it takes some time and experimentation.  I guess its the “self-sufficiency” in me.  So, here we go – .303 British Brass converted to .410 shot shells.

I had a bag full of once-fired .303 British brass given to me so I elected those cases to be the victim of experimentation  over some of my brand new, unfired supply of .303 brass.  Some of the cases had bulges just above the rim from an obvious head-space problem caused by it’s former firearm.  After, checking the cases for preliminary case-head separation, discarding the obvious problematic cases, it was time to start depriming the salvageable cases and began the conversion by tumbling the brass to clean them up.

The first hurdle to overcome on the conversion is to resolve an obvious problem with head-space and the .303 British case incapable of chambering in the .410 shotgun.  The problem being the .410 shot-shell has a rim thickness of .063″ with a 55 degree tapered rim and the .303 British having a .060″ untapered rim.  Despite the fact that the .303 British rim is thinner than the .410’s rim, the .303 British simply cannot fit in a .410 chamber without the taper. One way to resolve the problem is to mill a taper on the underside of the.303 British rim to make it similar to the taper of a .410 shot-shell.  Problem is, I don’t have a milling machine – but why not just file a few thousands off the case head of the .303 British so it seats flush with the .410 breech.  Let’s do it.

I started by securing each case, one at a time, in a padded vice and began filing evenly across the case head.  Initially, I was unsure as to how exactly much of the case head needed to be thinned so I started by counting the passes with the file and counted 30, 40, then 50 passes and tested the brass for a fit in the chamber until the action closed on my single shot shotgun.  After an average of 50 passes with the file, and entirely removing the lettering on the case head, it was found that a rim thickness of ~.055″ would fit flush and allow the shotgun to lock up.  To the right is a comparison of a filed case and an unfiled case.  Make sure that when you file the case you keep the file level, squared with the case head so as not to create an uneven surface on the face of the rim.

Next, I test fitted the .303 British case against the .410 breech to ensure a flush chambering of the brass was possible.  Then, I simply closed the action of the single-shot .410 shotgun with the case in the chamber and it fit perfectly.  Since I’m using a single-shot .410 it was rather simple to do; however a pump or semi-auto shotgun may involve more precise fitting and workmanship.  So basically, instead of removing metal from the underside of the .303 British rim to thin (or taper) the case, we simply just removed metal from the top of the rim (see below). 

One concern I read regarding the thinning of the case head by using a file was that, thinning the case head will make the primer seat above the head of the case.  Well, that simply isn’t true.  As you can see from the photo that removing only a few thousands of an inch that the primer stills seats well below the face of the case head where it belongs.

Now we begin to fire-form the case to make the tapered .303 British case a straight walled .410 shot-shell brass hull.  Since we have to fire-form the majority of the case to get it to become a straight-walled shot-shell, we’ll do this process in no less than two separate fire-forming procedures.  The first being to blow-out the neck and shoulder and the second to make the rest of the case straight.  We start by annealing the cases with a propane torch, a cold pan of water and a spark-plug socket chucked into a cordless drill.  Spin the case using the drill to help in uniform annealing.  As it spins you can see the color change as the metal heats up and works down the body of the case – do not, nor is it necessary, for the case to turn cherry red while annealing. Once the case turns a bluish color drop the annealed case into the water.  Since we’re moving a lot brass during this fire-forming I am annealing more than just the case neck and shoulder; in fact about 2/3 of the case is annealed; being certain not to over heat the lower 1/3 – that portion should not even begin to change color – and it helps to be protected by the socket and prevent over heating.  After the cases are dropped in the water they become pretty soft and malleable.  Place the cases aside to dry thoroughly before priming.

Once the cases have dried they were primed with large pistol primers.  Since shotgun shells have very low pressure levels, lower than pistol cartridges, it makes sense to use pistol primers versus rifle primers.  I used Wolf large pistol primers as they are very reasonably priced compared to their U.S.counterparts and serve this purpose very well.

For the first fire-form, I load the .303  brass with 12 grains of Red Dot and pack it full of Cream of Wheat.  Fill the case up to about 1/4″ from the top of the case – then pack the Cream of Wheat.  Then cover the last 1/4″ of space with wax or other substance to keep the Cream of Wheat from falling out.  Load and shoot.




As you can see from the first fire-form effort the upper half of the case expands to fit the .410 chamber and the shoulder is completely blown out to form a nice straight case neck.  Now that the neck and shoulder is expanded to a rough .410 shot-shell we can start loading for a second fire-form to help expand the lower section of the case. 

After seating new primers in the cases for the second fire-form I loaded 12 grains of Alliant 2400 powder then inserted a .410 factory wad (shown are Claybuster CB5050-410HS wads).  When inserting the wad you may find that the wad does not fit entirely inside the case because the lower, unexpanded section, prevents the wad from fitting all the way in.  No problem, just trim the fins on the wad and insert it into the case.  Another alternative to plastic wads is to use a .410 over-shot card and a fiber cushion wad cut to the necessary diameter.  I actually seen one individual stuff it with T-shirt material and shoot.



Next, I filled the case with approximately 1/2 ounce of lead shot.  Then, using either a .45 diameter punch or a .45 ACP empty case with the edge sharpened I cut an over-shot card to place over the shot. I then pressed the card in place with a dowel and then covered the over-shot card with Elmer’s glue to hold the card in place.  After the glue dried I loaded them up and fired away.

You can create a niceroll crimp on the shell using a .308 Winchester bullet seating die as an alternative to gluing the card in place.  I chose not to do this since I don’t want to work the brass anymore than necessary – but it is an option.

To the right we see the results of the second fire-form.  Not too bad if I say so myself.  Might need one more 1/2 ounce load to get the final needed expansion at the base but most certainly loadable.

I need to note that the brass case is thinner walled than the plastic cases.  That said, you may need to modify your choice of wad column diameter.  What I’ve done is to cut a 1/8″ over-powder card the diameter of .460″ to place over the powder charge to ensure a seal, then place a plastic Claybuster wad on top of that, fill with 1/2 ounce shot, then cut an over-shot card out of cereal boxes at .460″ and seal it with Elmer’s Glue.  The punch I use is the same one used to cut wads for my .44 black powder pistol.  Punches are available at Harbor Freight for about $8.  And finally, if you find your brass sticking in your chamber and need to resize the cases you can place the shell in a .303 British shell-holder and run it up into a .45 ACP sizing die and size about 5/8 of the case for a smooth chambering.

Wrapping up I wanted to comment that I read in an older reloading manual that when it comes to brass shot-shell cases that, “Not going on much here.  If you have any of these, put them up on a mantle and think up a good “Grandpa told me” story to tell people when they ask, “What’s that?”.  I got a better idea – why not show those people how to make, load and shoot these old brass cases and carry on the legacy of our Great Grandparents…I bet you’ll have more fun actually doing it than talking about it.

Safe Shooting!




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