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Wildcatting: Slam Dunk

Wildcatting: Slam Dunk

How much power do we really need for deer, especially at the ranges most woodland species are shot? Designing something that offers moderate recoil and therefore increased control and is easy to moderate and effective at the target without excessive carcass damage is a serious thought.

Many cartridges fall into this category but one that instantly springs to mind is the Bench Rest case or BR originally designed by Jim Stekl of Remington Arms in the mid 1970`s.

The mid size BR or ½ size .308 case is coming into its own, not from its original 7mm calibre configuration although good but from its many forms of wildcatted sizes starting from .17 all the way up to .35”.

Not only will this case size produce enough energy to be deer legal with bullets ranging from 160 to 250-grains but you also have the opportunity to load bullets from 250 to 300-grains at subsonic levels for some real fun. I am a bit sad when it comes to subs and this .338 BR is seriously good to shoot.


Because it shares the same head size as the .308 any rifle thus chambered can be re-barrelled to shoot the 338 BR. However being short and stubby, magazine feed can be an issue! Therefore you might want to buy a custom action build for the .30 BR, or 6mm BR and modify the magazine to work properly.

I love the old Tikka M55 and M65 actions, which represent old world quality that newer rifles just do not have, they are all metal and fantastically hard wearing. I sourced an old M65, from Gregor Mcleod of Tain;  a M55 short would be better but this M65 long action actually was a Sporter with magazine blocked to feed .308 rounds

The barrel was a stainless steel Border Match grade tube, ordered at 30” but cut in half for two rifles because I wanted a short barrel. It was ordered as a blank that Steve Bowers profiled to match the receiver of the M65 to a heavy short 16” length with 14.5” of rifling. Twist was very important if I wanted to utilise the 338 BR as a subsonic, so the traditional 1 in 10” rate was not really good if I was going to use 300-grain bullets. So I selected a 1 in 8” rate which would be fine also for supersonic use. I kept the stock the same and Steve worked the trigger and I fitted a custom-build MAE 38mm sound moderator.


Water capacity of a .338 BR case was 42.5 grains with Lapua 6mm BR brass and with a 250- grain bullet seated you have 25-30 grains of powder capacity for a non compressed load. This would be with a relatively fast powder achieve supersonic velocities in the 2200fps and over 1700 ft/lbs range. With a 250-grain bullet deer legal in England and Wales with 160 grainers you should achieve the 2450 fps velocity minimum for Scotland!

Case forming involves expanding 6mm BR Lapua or Norma brass up in stages to .338” with K&M mandrels supplied from Sinclair International. Take your time and lube the necks well until the final stage where a degree of neck turning is essential to eliminate a forming bulge at the base of the neck, this done just reload and fire form the brass as usual.

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The .338 BR is a small case and with a 160-250 grain flat base bullet loaded the OAL was only 2.299 to 2.474” which meant the bullet was seated deep so reducing powder capacity to 25-30 grains. This was quite handy, as relatively fast powders of small weights are necessary to launch a bullet to deer legal 1700 ftlbs energy from only a short 14.5” barrel. Quickload, a computer ballistics program supplied from JMS Arms, suggested VIT N120 with a charge weight of 26-grains maximum to achieve 100.1 % load density, 100% burn rate and the magical 1827 fps/1854 ft/lbs, bingo! Alternatives would be Benchmark a medium burn rate that at 29-grains would give 1815 fps/1829 ft/lbs, or the slightly faster burning Alliant RL7 and RL10X.(See tables below)


Regarding ballistics downrange, this was conceived as a short range, low meat damage tank buster round not a high ballistic coefficient long range cartridge. Therefore the trajectory out to 200 yards is, yes a little rainbow-like. But when zeroed at 30 yards (close range) a 250- grain round nose Hornady is +1.6” high at 100 yards, -1.2” low at 150 and at long range, in these terms -7.8” low at 200. But it really ‘thumps’ when it hits and causes far less hydrostatic shock or blobbed/bloody mess on the carcass when you come to gralloch it, in the words of Elmer Keith “ You can eat right up to the hole” (oh ahh)...

The sub-sonic has no legal deer use at all and are only shot for fun, to me that’s long range sniping at steel silhouettes. Steve made me a 120 MOA angled scope mount for 500 yard subsonic sniping. It looks really weird but you need it, as that big 300-grain slug drops like a stone at range.

Sierra 300 gr Match Kings were outstanding but are very expensive especially when bought in 500 bulk packs. So I switched to 250-grain Lapua Scenars which were just as good. It’s an odd thing shooting a 338 BR subsonically as with the MAE fitted all you get as a muzzle signature is a ‘throoong’ sound as the metal rattles and stiffens a little, no muzzle blast at all.


Then 1.5 seconds later at 500 yards the strike is heard. It’s very addictive to shoot and you soon realise that large sub-sonic calibres may be travelling slowly but because they are below the speed of sound then there is little or no wind resistance and the bullets just cruise through the air and take on little wind, rather like an old muzzle loading bullet.

Ballistics down range are thus; a 250-grain Scenar with a healthy 0.675 BC and 1089 fps still has 933 fps at 500 yards, only a loss of 156 fps and the energy from start is 658 ft/lbs is still 483 ft.lbs at 500 but the trajectory -328.9” at that range. Interestingly though is that in a 10 mph wind it has only drifted 16”.

Accuracy is astonishingly as with a lot of big subsonic rounds. At 100 yards I can get all three shots touching, near enough. At 500 I use Archery targets to stand out well and you can achieve 8-10” groups with 10 shots. The only problems as always with subs in centrefire`s is barrel fouling which can cause a sudden and dramatic velocity drop and therefore a miss by feet, not inches!


This is an easy round to make and mimics the 338 Whisper in many respects and is equally at home in the woods with supersonic heavy bulleted loads or tumbling steels at 500 yards with sub-sonics.

The meagre powder charge is very efficiently turned into useful velocity, there is little or no meat damage, in my Tikka it hardly kicks and with a MAE sound moderator is beautifully quiet. I have taken many deer with the 338 BR and I love shooting it as its short length is very handy in the woods although those big old bullets whistle right through a Roe so extra care is needed with a safe backdrop as with the subs as those big slow bullets ricochet like Wild West films sound effects!