The 8mm ARP (Andrew Roy Pridham)

My motivations for this were:

- I like the 8mm bore size – no explanation, no apologies
- an interest in making use of commonly available and cheap 8mm milsurps
- I have grown a bit weary of my research on the 8x63mm Swedish
- I wanted to design a magnum wildcat that would match or exceed the capacity and performance of the 8mm Rem Mag, but in a standard (30/06 length) action
- it had to use readily available and inexpensive brass (I use 300 or 338 RUM)
- the forming and resizing process had to be as simple as possible and inexpensive.
- the case would have a 35 degree shoulder (same as 325 WSM), a calibre length neck and would take into consideration some interesting Bench Rest cartridge theories for efficient gas flow Article on Shoulder Angles and Lengths
- the case dimensions and throat length would allow bullet weights from 180-250gr to be seated close to the lands, but still be fully gripped by the neck at an OAL of 3.420". Throat length is a short 0.125", but could be lengthened with a throater for heavy bullets if OAL restrictions are ignored. Given that case capacity is about 105gr of water, heavy bullets impinging on powder capacity is not a big concern.
- I have an alternate design for the same cartridge at 2.54" vice 2.62" long so as to fit in Mauser actions designed for the 8x57 and its 3.25" OAL. I would use the same reamer with a shorter go-gauge (which I have) to chamber, and the same case forming tools and process

The 35 degree shoulder does not pose feeding problems with the Mauser controlled feed claw, but ejection is a bit weak and needs some work.

Cartridge Drawing

Many will say "why?", or the "338 Rem Mag is better", or "bullet selection sucks", etc. I did it because I wanted to, and I have learned a great deal.

I want to keep pressure under 60K. I have some experience developing loads for the 8x63mm Swedish, and of course a chrony. If I am, for example, seeing much more than 3100 fps with 220 gr bullets, then that should be a clear sign that pressure is at or close to max.

I bought a reamer and go gauge from Pacific Tool. The test gun is based on a military K98K Mauser action with a military 28" barrel (with 1:9.5 twist, fast enough for even the 250 gr Woodleigh). The bolthead was opened, a properly bent handle was installed, the mag box opened to the max (3.420â€), feed rails and ramp tuned, and scope mounts added (it was already bubba'd). Jason Spencer at Gunco in Ottawa did the work, and I am completely satisfied.

Test Gun

Test Gun Receiver

Here's how I form the brass:

1. Anneal the top 0.75" of the 338 RUM or 1.0" of the 300 RUM brass
2. Run it through a Lee 8x50R Lebel F/L die, set so that the neck/shoulder juncture is close to its final distance from the base (2.300" in this case). The 8x50R die has a nice gentle slope and the correct 8mm neck. I use the excellent Redding Imperial Sizing Die Wax, which can be bought from Wholesale Sports.
3. Trim to 2.620" (I use a Lee 300 Win Mag trim die)
4. Fireform using 20.0gr of Unique under Cream of Wheat packed under a bit of tissue
5. Deprime
6. Neck size using a Redding 325 WSM F/L die (big enough that it only works the neck and has the correct 35 degree angle)
7. Body size just below the neck back to 0.5320" (when necessary) using a Lee 338 RUM F/L die shortened 0.500†and with the decapper removed
8. Load using the 325 WSM seating die.
9. Inside neck ream after the first firing
10. Repeat steps 5-8 for each subsequent firing

Fireforming Sequence L to R: 300 RUM, Annealed and Sized, Trimmed, Fireformed

All I needed was the Lee 8x50R Lebel F/L die (which I already had), the Redding 325 WSM Sizer die and a Lee 338 RUM F/L die set - total cost less than $100. I have since purchased a custom "Collet Die Set" from Lee, which neck-sizes and seats bullet, and the $55 was some of the best money I have ever spent.

Loads:

The 8mm ARP holds 105.0 gr of water full to the top. The 8mm Rem Mag holds about 98.5 gr, and the 340 Wby Mag, about 102.0 gr. Logic and experience indicates that my max loads should fall between the two, i.e. greater than the 8mm Rem Mag, but less than the 340 Wby for all bullet weights and powders (in fact given the practice of Wby loads being on the high side of safe pressure and their usual use of a long throat, I would be wise to stay well beneath it).

My starting loads used 8mm Rem Mag max loads plus about 3% (i.e. about 2 grs) and I stayed at least 4% below 340 Weatherby max loads (i.e. about 3 grs). I can't trust the classic high pressure "signs" (primer flattening, stiff bolt lift, etc.), and will work up an accurate load anyways, but as with any wildcat, am left to my own devices to determine max loads.

Here’s my first round of results (velocity only) and my analysis of them:

8mm ARP, Max cartridge OAL 3.420†(0.080†off the lands), 28†barrel, chrony 20’ from the muzzle, temp 12 deg C. All loads were at about 95-98% load density (which I think is too high, the 90% range being better). CCI Large Rifle Magnum primers. Velocities not adjusted for distance, temp or humidity.

125 gr Speer – OAL 3.275†– 89.0 gr RL19 – 3210 fps – powder too slow (try RL15 and H414),

150 gr Speer – OAL 3.350†– 93.0gr RL22 – 3530 fps – powder too slow (try RL19 and H4831)

170 gr Speer – OAL 3.350†– 91.0 gr RL22 – 3450 fps – powder about right

180 gr Nosler – OAL 3.420†– 90.0 gr RL22 – 3450 fps – powder about right

200 gr Nosler – OAL 3.420†– 91.0 gr RL25 – 3170 fps – powder too slow (try RL22 and H4831)

220 gr Speer – OAL 3.420†– 89.0 gr RL25 – 3070 fps – powder too slow (try RL22 and H4831)

See the website below.

Very Nice! roger

Thanks for the info! That would make a fine round in .416" as well. Good job. Thanks for sharing.

Excellent. I too think 8mm is generally underated...Because of budget constraints, I'm probably gonna build 8mm-06 AI. But won't get the kind of performance you're getting. Eagerly awaiting more info!

Very nice!

dfaugh,

The 8mm-06 AI is a great round and a very straight-forward and inexpensive conversion.

At least one other guy on this board has posted loads for it in the past, and you could go to my website and use some of the 8x63mm Swedish loads minus 5%.

Andy -

Note that your expansion ratio is a dead ringer for the 7mm Mag which explains why some of the early powder selection were too slow. Nice work and good luck!

Andy,

Very excellent work! This looks like the king of 8 mm that will fit in a standard Mauser action. This is VERY cool!

I'm curious about how you tell the powder is too slow. Is muzzle flash the evidence?

Thanks again

quote:

Originally posted by DobleTroble:
....I'm curious about how you tell the powder is too slow. Is muzzle flash the evidence?....

A good question. I should have said "Might be too slow". I based this on the fact that the case was full, pressure was ok (no casehead expansion of other symptoms) and velocity wasn't quite what I had expected based on what I got with other bullet weights. As it turns out, RL25 is a pretty good powder for the heavy bullets, but the original powders for the light bullets were "too slow", i.e. not maximum velocity at safe pressures. Here's the revised list:

125 gr Speer – OAL 3.275†– 89.0 gr RL19 – 3210 fps – powder too slow
125 gr Speer – OAL 3.275†– 86.0 gr H4350 – 3440 fps – powder a bit slow
125 gr Speer – OAL 3.275†– 85.0 gr H414 – 3650 fps – powder about right

150 gr Speer – OAL 3.350†– 84.0gr H4350 – 3370 fps – powder a bit fast
150 gr Speer – OAL 3.350†– 89.0gr RL19 – 3400 fps – powder a bit fast
150 gr Speer – OAL 3.350†– 93.0gr RL22 – 3530 fps – powder about right

170 gr Speer – OAL 3.350†– 91.0 gr RL22 – 3450 fps – powder about right

180 gr Nosler – OAL 3.420†– 90.0 gr RL22 – 3450 fps– powder about right

200 gr Nosler – OAL 3.420†– 91.0 gr RL25 – 3170 fps – powder too slow
200 gr Nosler – OAL 3.420†– 87.0 gr H4831 – 3180 fps – powder a bit fast
200 gr Nosler – OAL 3.420†– 88.0 gr H1000 – 3120 fps – powder a bit slow
200 gr Nosler – OAL 3.420†– 86.0 gr RL22 – 3230 fps – powder about right

220 gr Sierra – OAL 3.420†– 89.0 gr RL25 – 3070 fps – powder about right
220 gr Sierra – OAL 3.420†– 85.0 gr H4831 – 3010 fps – powder a bit fast
220 gr Sierra – OAL 3.420†– 85.0 gr RL22 – 3100 fps– powder a bit fast
220 gr Sierra – OAL 3.420†– 87.0 gr H1000 – 3010 fps – powder a bit fast
220 gr Sierra – OAL 3.420†– 90.0 gr Retumbo – 3050 fps – powder about right

just wondering what stock that is on your test rifle? i kinda like it...for plastic...

How does this compare with say an 8mm/.338 win Mag.

It has about 15% more case capacity than the 8mm/338 and at equal pressures will realize about 4% more velocity than it for any given bullet weight (i.e. 125-150 fps).

If you fall prey to "wildcat insanity" (as many do) and add powder until you blow primers or have to hammer the bolt open, 300 fps would be easily achievable.

Andy, I saw your post over on the Reload Bulletin Board. I thought I would answer here.


Looks like you have a nice wildcat there. I sure do like your velocity data that you posted!


My design is made off the .404 Jeffery parent case. It has a case capacity of 103.3 grains of H2o. I use either .300 or .330 Dakota cases to form as that is the easiest way and doesn't involve too much forming and annealing.

I just got the rifle together as I first had to finish with DuraCoat. Here is a few pics of the rifle. It is a Remington 700 Classic, 26" 3 groove Pac-Nor barrel.




I haven't any data to share at the moment as I had rotator cuff surgery the end of August and am not cleared to go to the range for some test firing. I did manage to get some brass fireformed with my Son. The way I designed the case, I can just fireform with bullet as I get a crush fit much like the Ackley designs. The head is .545" and the shoulder is .536" OAL is 3.310" with a 35 degree shoulder.

Good work.

Grizz,

Great work, and very nice gun - contrasts nicely with my "Beater" and shows two approaches. I doubt you spent much more on yours than I, but I wanted to show how you could use an old milsurp gun.

These wildcats do fill a void in the thousands of cartridges out there by providing 8mm Rem Mag performance in a 30/06 length case. I was surprised to see in the latest Handloader mag a story on a "new" 9.3mm wildcat and a statement to the effect that 9.3 was essentially the only remaining hole to be filled.

I found the effort enormously satisfying and educational. Neither I nor the shooting community "needed" this round, but nevertheless I think that posting our results is of great benefit.

I will soon have a one-page website up http://www.8mmARPWildcat.pridham.ca and I will register it with at least Google (8mm, Wildcat, etc.). I encourage you to so the same.

In the world of wildcatting, we should be careful to never claim to be the first, but we can confidently claim to be the first to publicize!

Andy,
when the rum came out, i thought "that would be perfect, about 2.65" long "

good work.

my only complaint is that you didn't do this in .358!!

jeffe

Andy, I will take your suggestion about starting a website on the wildcat. That is something I never thought about.

Again, nice work and hope I get some of the velocities you did!

Have you tried AA MagPro?

Jude

quote:

Originally posted by jeffeosso:
Andy,
when the rum came out, i thought "that would be perfect, about 2.65" long "
good work.
my only complaint is that you didn't do this in .358!!
jeffe

Hey, Jeffe, What would you do with a 2.65" 358 case? Seems like a lot of bullets would have noses too long for it, assuming 3.34" COL.

bb,
the 550 express is a 2.65" case with a 3.35 max oal...and the bullets sit about 1 caliber in!!

this would be a large gain over the 358 norma, with the same (nearly) oal.

jeffe

I don't follow you.
If the case is 2.65" and the bullet is in .35", you have a 1" bullet, which is pretty small IMO. And you only have room for round noses; spitzers of any size would go into the neck.
Might be great for 220g FN, but that's alot of powder for that, isn't it? Open up the action to 3.45" or so, and you've got all the room you need, but then you're awefully close to a 35* RUM at that point.
If you kept the same axial dims as the Norma, you'd see a pretty big increase anyway. About the same powder room as the STA. And room up front for at least a 6-caliber ogive, maybe 8. The, uh, 35 Howel or something (not the 30-06 "long" round) was on the 404, at 3.34" - basically a 35 Dakota, and it does similar to the STA I hear.

BB,
most 35 caliber bullets ahve the cannelure around .7" from the nose.. not a problem in this situation.

1) this is awesome
2) I want one

I think Charles Askins did this about 50 years ago. I don't believe I ever saw any case dimensions but it was an 8mm based on the .404 case. As I recall he took it to Africa and was pretty disappointed with its performance. At the time I don't think he had any good bullets to play with.
C.G.B.

cgbach, reading about how good the .300 Dakota and .330 Dakota is on anything on the North American Continent and reading about how well they actually did on many African game, I can't help but think an 8mm off-shoot can do the same or better considering that it holds 5.5% more case capacity than a Dakota variant.

Yes, I know that some areas of Africa have caliber limits that an 8mm is not allowed. My intentions as mentioned was to meet or exceed the 8mm Remington Mag.

Andy, my congatulations! this a very sound proposition, from every angle I can see it.

In fact, I was thinking about a similar cartridge, but of .30 cal instead of 8mm.

However, your choice seems to be a very good one. Not so many europeans could be wrong with their 8x68mmS...

Only one thing worries me ... the 28" barrel.

Have you ever think of testing the performance at 24" ??

tks!

A 24" barrel is more practical in most applications, but once I cut it off, I can't put it back.

I guess it would lose 100-125 fps going from 28" to 24".

Sounds like a heck of a rig Andy. I've been in a similar boat lately trying to work up loads for an 8mm/378 Weatherby on loan from my father. It's built on an old BRNO (CZ) action originally chambered for 8x68S. The accuracy out of this gun is really astonishing considering what an ugly pig the gun is. You velocity results look like they're right on the mark too.

As of late, the best results I've been getting were with Reloder 22. I tried some Retumbo but it wasn't as accurate. I also tried H-4831, and H-1000, both with less than stellar results. With 102 grains of RL22 I can push 200 gr. Nosler Accubonds (big thanks to Nosler for putting out a nice bonded bullet in 8mm) at 3325 fps. Considering the gun only has a 25.5" barrel, that's pretty respectable. Good luck in the future with your project. Hopefully with Winchester pushing the 325 WSM the 8mm bullet selection will improve but in the meantime, I'm very pleased with the way the Accubonds are shooting.