Heavier projectiles in the 7mm-08By Matthew Cameron
- 24th Feb, 2020 Feb 24, 2020, 2:11 PM
- 10 Comments
For various reasons I had disposed of the only 7mm rifle in the family gun safe and the space in the rack needed to be filled. Not a bad excuse for purchasing another rifle.
There’s nothing flash or exotic about the 7mm-08 cartridge. When Winchester released the parent .308 case in 1952, it wasn’t long before experimenters necked the case down to accept 7mm projectiles. Remington “legitimised” the dimensions in 1980 and the shoulder angle remains the same at twenty degrees, but there’s also an Ackley Improved version with a 40 degree shoulder.
The factory originally offered the 7mm-08 loaded with 120 grain and 140 grain projectiles, however American writers have suggested that heavier projectiles might be more effective, but that depends on what you are aiming at. Others have compared the capacity and performance of the 7mm-08 with the 7x57 Mauser, but they’ve also suggested that the 7mm-08 really needed a bit more powder capacity.
“...experience with the 7x57 at typical velocities showed that the 175 grain projectiles were very efficient, particularly in thick scrub. I could not see that the 7mm-08 would be any different.”
I think the Americans have forgotten that the 7x57 made its name in the game fields of the world using 175 grain projectiles at a modest 2300/2400fps, and many of the animals it dropped successfully were much bigger than the pigs and deer I had in mind.
My experience with the 7x57 at typical velocities showed that the 175 grain projectiles were very efficient, particularly in thick scrub. I could not see that the 7mm-08 would be any different. With modest powder charges the 7mm-08 appears to have inherited its .308 ancestor’s inherent accuracy, but with less recoil. I reckon it’s another “balanced” cartridge where case and projectile seem to fit well together.
CHOOSING A HUNTING RIFLE
The question was which rifle to purchase? In the end I decided on a compact model Ruger American – the price was reasonable and US reviewers considered the Ruger good value for money, although that was only one of several criteria. I liked the idea of an 18-inch barrel in bush or thick scrub for fast target acquisition.
In addition, the smaller rifle would be more suitable for introducing my grandchildren to hunting. In the meantime a simple slip-over recoil pad would increase the length of pull to accommodate adult shooters. The accuracy would be interesting to check out - all the reviews suggested that these rifles were in hunting terms, very accurate, with three shot groups of an inch or less common. What’s more, my local gun shop had one in stock!
AT THE BENCH
Now I needed cases, reloading dies, and also a form and trim die to handle case length. It would be interesting to see exactly what was available locally as all the major U.S. reloading companies make dies in the calibre.
I soon found that the normal form and trim dies make case length a non-issue, simply run a lubricated case into the die and file off any excessive case length, then chamfer the neck in the normal manner, problem solved. Much quicker than some of the small lathe-type turning tools that have to be reset for each calibre.
The choice of powder was easy also. Perusing ADI’s 7th edition Handloader’s Guide I found a range of suitable powders from BM2 to AR 2213SC, including AR 2208 and AR 2209, all of which I already had in my powder cupboard. I figured AR 2208 would be the most suitable for the 140 grain projectiles, with a tossup between AR 2209 or AR 2213SC for the heavier ones.
The only way to find out was to create some test loads and shoot for results. My preference is for what I call “middle of the road” powders, although my reloading notes show that the best accuracy is sometimes provided by powders at either end of what is recommended.
Some cartridges seemed to produce better accuracy using powders a bit on the slow side of those recommended in various manuals, others faster. Perhaps barrel length enters the question here, faster powders being more efficient and burning quicker in a shorter barrel?
In addition, loading density (ie; the amount of space the powder load takes up inside the case) may have a large influence on accuracy. There was another factor I hadn’t considered. Keeping to a cartridge overall length of 2.8 inches the longer and heavier projectiles would fit deep into the case, hence reducing powder capacity, a situation that deteriorated when I discovered that with a maximum length of 2.8 inches the 170 grain Sierras wouldn’t reliably feed from the magazine due to the blunt nose of the projectiles.
Several dummy loads showed that reducing the overall length to 2.7 inches solved the problem, but this also reduced the powder capacity further.
My initial problem was the lack of reloadable brass, but I eventually bought a box of fifty cases. I prefer not to use top quality brass for hunting, bearing in mind the inevitable loss rate, however the upside was that I would probably be able to reload them an additional number of times. I could have bought factory ammunition and reloaded the empty cases, but as a dedicated reloader this was my second preference.
--- Article continues below ---
A third option was to neck down .308 brass. A mate provided some ADI 1993 once-fired .308 cases, from which he had actually removed the military crimp, then reamed out the primer pockets. The lubricated cases were passed through the trim and sizing die and then a normal resizing die. All of the cases were then annealed.
The choice of scope was also a consideration. I did have an ancient 6x40 Japanese model that would do the job. Oddly enough when I purchased this sight many years ago I didn’t consider it to be of very high quality, yet it has now spent thirty years on several rifles without fault.
PROJECTILES AND LOADS
For each projectile weight I consulted six reloading manuals, including ADI, and using the IMR equivalents the loads were averaged out. I was seeking the most accurate loads. For the Nosler 140 grain boat-tail the loading density suggested that AR 2209 might in fact be the better overall powder.
The maximum averaged load was 47.5 grains, and three descending loads of one grain less each completed the initial bunch. The top load of 47.5 grains filled the case to the bottom of the neck, although using a drop tube lowered this level slightly. The loading density with the bullet seated to maximum length appeared to be close to 100% or perhaps just slightly compressed.
The combination of the 140 grain Ballistic Tip projectiles and AR 2209 produced the following: See Table 1.
I believe that the 1.43 inch group was an aberration possibly caused by my handling of the excessively heavy factory trigger.
|TABLE 1: 140 GR Ballistic Tip projectiles + AR 2209|
Normally I log any pulled shots but this was not the situation with this group. Several weeks later my two proof maximum loads averaged 0.89 and 1.1 inches, for an overall average of 0.95 inches, and velocity overall averaged 2640fps, all very acceptable. This maximum will be my first choice for hunting. Bolt lift was normal with all loads and there were no other excessive pressure signs.
With the 175 grain Hornady spire-points AR 2208 would provide more powder space with a maximum averaged load of 37.5 grains, and three other loads in descending order to a low of 34.5 grains would provide an indication of what the powder was capable of. Another reason for testing AR 2208 was its slightly faster burning rate, ideal in the shorter barrel.
As mentioned, I was happy with achieving hunting accuracy. It would be nice if the groups were in the half-inch range but it was not necessary and probably unlikely. Initial results were: See Table 2.
|Table 2: 175 GR Hornady soft-points + AR 2208|
Including the two proof loads the overall averages for the maximum load were 2228fps, and the groups were well within the accuracy required for hunting, with no signs of excess pressure.
Three of these initial loads were sub minute of angle, but again there was one aberrant group. At this stage I suspended further testing until the trigger was lightened up a bit. Even allowing for the one large group I was impressed with the Ruger’s overall accuracy.
I thought it was very reasonable from an 18-inch lightweight barrel. Twenty or so years ago accuracy like this out of a factory rifle, let alone one fitted with a very slim barrel, would have been unusual if not unattainable.
“Overall the 7mm-08 did not seem to be fussed about powders or the longer projectiles; perhaps like its .308 parent it is a somewhat flexible calibre.”
I thought that perhaps AR 2209 might also be suitable with this 175 grain projectile. The short story is that the loads were once again compressed and the groups were all over the place. The maximum proof loads resulted in a hard bolt lift and an ejector mark on the case, clearly a case of high pressure and an obvious overload.
The question was, why? The original maximum load did not show any signs of pressure, so I disassembled the remaining proofs to check them. All the powder charges were correct. I am still unaware what caused the overpressure - did I load the wrong powder? The mystery remains, but events like this make for extra caution at the bench.
--- Article continues below ---
The question of compressed loads is an interesting one, how much is too much compression? Does it vary with case design? Some of the information available suggests that over compression can lead to pressure spikes, and perhaps affect accuracy?
To get a full load into a case and minimise compression, Graeme Wright (Shooting the British Double Rifle) suggests using a drop tube to settle the powder lower in the case, and pouring the powder into the funnel very slowly. I employed both of these techniques.
With the 170 grain Sierra round-nose projectiles and AR 2209 I averaged out four loads commencing at 42 grains to a maximum of 45 grains. I use AR 2209 in several hunting calibres - it’s a very flexible powder and almost always delivers good results. Maximum loaded length was 2.70 inches, ensuring that all cartridges would feed smoothly from the magazine.
The initial results with AR 2209 were a best group of 1.1 inches and a worst of 1.76 inches with an average of 1.34 inches overall. Three of the four groups had a single flyer, and I noted that the lowest charge produced the best accuracy, maybe that was due to less compression? Another possibility was that the barrel twist rate might be unsuitable, but consulting Load From a Disc showed that this assumption was incorrect.
Now I changed to AR 2208. With a maximum load of 40 grains the 170 grain Sierras produced the following initial results: See Table 3.
|TABLE 3: 170 Gr Sierras + AR2208|
This gave an overall average of 1.23 inches. It was interesting to note that the initial best group with both powders was one grain below maximum. Acceptable, although I was still of the opinion that there was a better load.
According to ADI’s latest manual another possible powder was BM 8208. The initial results were very encouraging. See Table 4.
|TABLE 4: 170 Gr Sierras + BM 8208|
The proof loads of 35.5 grains produced a two group average of 1.4” with velocities just over 2200fps, certainly within hunting accuracy limits, and I doubt that any deer would notice the difference in velocity. It’s all a question of overall balance; speed is useless if the projectiles are sprayed all over the landscape.
Next I tested some 140 grain Nosler Accubonds, proven deer medicine in other calibres. The ADI manual showed a maximum load of AR 2208 to be 42.2 grains, so I tested the following: See Table 5.
|TABLE 5: 140 Gr Noslers + AR 2208|
The proof loads averaged 2659fps and 2654fps respectively with group sizes 1.9 inches and 1.05 inches. Overall the velocity averaged 2660fps and accuracy 1.3 inches, again very acceptable to me as hunting loads.
Overall the 7mm-08 did not seem to be fussed about powders or the longer projectiles; perhaps like its .308 parent it is a somewhat flexible calibre.
More in Guns
More from NZGUNS
- 27th Oct, 2020
- 0 Comments
The Game Animal Council (GAC), the NZ Deerstalkers Association (NZDA) and Hunters for Conservation...