The 8mm ARP
I did this work in 2005, but have just now posted about it. I still have the gun, brass, etc. but have not shot it for about 10 years. Maybe this year..........
My motivations for this were:
- I like the 8mm bore size – no explanation, no apologies. Some might ask "Why?", or say "An existing round is better.", or "Bullet selection is poor.", etc. I did it because I wanted to, and I have learned a great deal.
- 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 the new 325 WSM), a bit less diameter at the shoulder than the 325 WSM (so I could use those dies as part of the process), and a calibre length neck. Final specs would take into consideration some interesting Bench Rest cartridge theories for efficient gas flow
- 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 a max OAL of 3.420". Throat length is set at a moderate 0.250", but could be lengthened with a throater for heavy bullets if OAL restrictions are ignored (and I am willing to accept the gun as a single-shot). Given that case capacity is about 105.0 gr of water (96.0 gr loaded with a 220 Sierra), heavy bullets impinging on powder capacity is not a big concern.
- I would make use of the "Ackley" principles of minimum taper and 35+ degree shoulder, to (allegedly) reduce bolt thrust, and to increase brass life.
- The design would prevent the 8mm ARP cartridge from inadvertently being placed in the chamber of either the 300 or 338 RUM (given that the headstamp will show one of those two chamberings). This was accomplished by reducing the taper thereby increasing the cartridge diameter from casehead forward to a size greater than that of either the 300 or 338 RUM. They could not even be forced in.
- 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 combination of 35 degree shoulder and minimal taper do not pose feeding problems with the Mauser controlled feed claw, but ejection is a bit weak and needs some work.
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. After fireforming, I measure the diameter of the casehead to establish a "Pressure Ring" diameter. I will not use loads that cause that diameter to increase by more than 0.001" - there is no better pressure indicator aside from an instrument (like the Oehler Model 43 - I wish I had one!), to indicate dangerously high pressure.
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 1:9.25" 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 (3.420”), 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.
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.
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. Seat the bullet using the 325 WSM seating die.
9. Repeat steps 5-8 for each subsequent firing. Inside neck ream after the first firing only
All I needed was the Lee 8x50R Lebel F/L die (which I already had), the Redding 325 WSM die set and a Lee 338 RUM F/L sizer - total cost less than $100. A set of Custom Dies would be a "Nice to have", but the "MacGyver Set" I use works well.
L to R: 300 RUM, annealed and sized using 8x50R Lebel, trimmed to 2.620", fireformed
L to R: Throat Length (0.250"), 220 gr Sierra seated to 3.420" (0.040" off the lands) showing how little powder space is consumed by the bullet
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 could fall between the two, i.e. greater than max loads for the 8mm Rem Mag, but less than max loads for 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, their usual use of a long throat, and the greater bore size of course, I would be wise to stay well below 340 Wby max loads.
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. That having been said, I intend to determine max loads (and hence explore its potential), but then back down to modest pressure, accurate loads. I am mindful of the high amount of bolt thrust this large diameter cartridge will deliver, and want to see relative long life out of the bolt, receiver and barrel. Accurate, below max 8mm Rem Mag #'s are just fine.
Here’s my first round of results (velocity only) and my analysis of them:
8mm ARP
Cartridge OAL 3.420” (0.040-0.080” off the lands for the 200 and 220gr bullets)
28” barrel (might add ~125 fps over a 24" barrel)
Chrony 20’ from the muzzle (farther than typical, but there's lots of muzzle blast!, might deduct ~15 fps over 10' from the muzzle)
Temp 15 deg C (I must be careful when I shoot and it's 30 C)
CCI Large Rifle Magnum primers
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 slow
200 gr Nosler – OAL 3.420” – 88.0 gr H1000 – 3120 fps – powder too 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
220 gr Sierra – OAL 3.420” – 104.0 gr US869 (c) – 3170 fps – powder about right
The 170, 180, 200 and 220 gr loads are max loads (as determined by some "signs" and common sense), and will be backed down by 3.0 or more grains as I settle on an accurate load. I would like to try Re33 and IMR8133 in the 200's and 220's, as well as more US869 in the same.
I did not achieve velocities I have heard that others have achieved with the 8mm Rem Mag and with shorter barrels, but have exceeded anything I have seen published for the 8mm Rem Mag. This is about the best I can get from my gun with what I am willing to load for it.
I did this work in 2005, but have just now posted about it. I still have the gun, brass, etc. but have not shot it for about 10 years. Maybe this year..........
My motivations for this were:
- I like the 8mm bore size – no explanation, no apologies. Some might ask "Why?", or say "An existing round is better.", or "Bullet selection is poor.", etc. I did it because I wanted to, and I have learned a great deal.
- 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 the new 325 WSM), a bit less diameter at the shoulder than the 325 WSM (so I could use those dies as part of the process), and a calibre length neck. Final specs would take into consideration some interesting Bench Rest cartridge theories for efficient gas flow
- 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 a max OAL of 3.420". Throat length is set at a moderate 0.250", but could be lengthened with a throater for heavy bullets if OAL restrictions are ignored (and I am willing to accept the gun as a single-shot). Given that case capacity is about 105.0 gr of water (96.0 gr loaded with a 220 Sierra), heavy bullets impinging on powder capacity is not a big concern.
- I would make use of the "Ackley" principles of minimum taper and 35+ degree shoulder, to (allegedly) reduce bolt thrust, and to increase brass life.
- The design would prevent the 8mm ARP cartridge from inadvertently being placed in the chamber of either the 300 or 338 RUM (given that the headstamp will show one of those two chamberings). This was accomplished by reducing the taper thereby increasing the cartridge diameter from casehead forward to a size greater than that of either the 300 or 338 RUM. They could not even be forced in.
- 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 combination of 35 degree shoulder and minimal taper do not pose feeding problems with the Mauser controlled feed claw, but ejection is a bit weak and needs some work.
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. After fireforming, I measure the diameter of the casehead to establish a "Pressure Ring" diameter. I will not use loads that cause that diameter to increase by more than 0.001" - there is no better pressure indicator aside from an instrument (like the Oehler Model 43 - I wish I had one!), to indicate dangerously high pressure.
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 1:9.25" 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 (3.420”), 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.
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.
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. Seat the bullet using the 325 WSM seating die.
9. Repeat steps 5-8 for each subsequent firing. Inside neck ream after the first firing only
All I needed was the Lee 8x50R Lebel F/L die (which I already had), the Redding 325 WSM die set and a Lee 338 RUM F/L sizer - total cost less than $100. A set of Custom Dies would be a "Nice to have", but the "MacGyver Set" I use works well.
L to R: 300 RUM, annealed and sized using 8x50R Lebel, trimmed to 2.620", fireformed
L to R: Throat Length (0.250"), 220 gr Sierra seated to 3.420" (0.040" off the lands) showing how little powder space is consumed by the bullet
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 could fall between the two, i.e. greater than max loads for the 8mm Rem Mag, but less than max loads for 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, their usual use of a long throat, and the greater bore size of course, I would be wise to stay well below 340 Wby max loads.
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. That having been said, I intend to determine max loads (and hence explore its potential), but then back down to modest pressure, accurate loads. I am mindful of the high amount of bolt thrust this large diameter cartridge will deliver, and want to see relative long life out of the bolt, receiver and barrel. Accurate, below max 8mm Rem Mag #'s are just fine.
Here’s my first round of results (velocity only) and my analysis of them:
8mm ARP
Cartridge OAL 3.420” (0.040-0.080” off the lands for the 200 and 220gr bullets)
28” barrel (might add ~125 fps over a 24" barrel)
Chrony 20’ from the muzzle (farther than typical, but there's lots of muzzle blast!, might deduct ~15 fps over 10' from the muzzle)
Temp 15 deg C (I must be careful when I shoot and it's 30 C)
CCI Large Rifle Magnum primers
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 slow
200 gr Nosler – OAL 3.420” – 88.0 gr H1000 – 3120 fps – powder too 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
220 gr Sierra – OAL 3.420” – 104.0 gr US869 (c) – 3170 fps – powder about right
The 170, 180, 200 and 220 gr loads are max loads (as determined by some "signs" and common sense), and will be backed down by 3.0 or more grains as I settle on an accurate load. I would like to try Re33 and IMR8133 in the 200's and 220's, as well as more US869 in the same.
I did not achieve velocities I have heard that others have achieved with the 8mm Rem Mag and with shorter barrels, but have exceeded anything I have seen published for the 8mm Rem Mag. This is about the best I can get from my gun with what I am willing to load for it.
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