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The 7mm Fatso - The Ultimate Short Magnum!

The 7mm Fatso - The Ultimate Short Magnum!

by Greg Duley
©Copyright 2009, NZ Hunter Magazine

The short magnum craze has taken the gun world by storm, with most manufacturers now producing rifles and ammunition for these cartridges. Based on the old British medium bore African cartridge, the 404 Jeffrey, these modern beltless cartridge designs are supposed to give you 2.5 inch case length medium magnum ballistics out of a short action, compact and lightweight hunting rifle. In factory ammunition they achieve this goal by loading the short magnums to higher pressures or by using the latest progressive burning powders. Handloading ammunition to the same pressure with the same powders in the same barrel length, the 7mm SAUM falls 100 fps short of the 1962 vintage 7mm Remington Magnum while the 7mm WSM drops about 50 fps short. When comparing the 300 short magnums with the 300 Winchester Magnum, they fall further behind due to the 300 WM’s 2.62 inch case length, which is longer than the other medium magnum's.

7mm Fatso
Jamie lining up on a sika hind at 700 yards in the Western Kawekas. I am on the 80mm Swarovski spotter.

7mm Fatso
And the result! You'll notice the muzzle brake is off for spook and shoot mode in the bush when you don't have time to use hearing protection. Quite surprisingly, the point of impact hardly shifted with the muzzle brake removed.

None of this is rocket science; it is simply due to case capacity. Despite the short magnums’ fatter cases, they still fall short of the standard length magnums’ powder capacity. This is not an issue with factory ammunition. In fact, Federal’s 7mm WSM velocities are on average 100 fps faster than their 7mm Remington Magnum ballistics with the same bullet weight. Obviously the ammunition companies are downloading the standard magnums to make the short magnums more attractive, so if you're purchasing a rifle to shoot factory ammunition, I know which one I'd be buying! With a shorter lighter rifle, better ballistics and a good selection of bullet weights and types, choosing between the WSM and the Remington Magnum for the factory ammo shooter is a no-brainer!

7mm Fatso
From the left: 7mm/08; 7mm SAUM; 7mm WSM; 7mm Fatso; 7mm STW; 338 Lapua Magnum; 416 Rigby. Based on a shortened and improved 338 Lapua Magnum, the 7mm Fatso gives 7mm STW ballistics in a short action.

Always trying to invent a better mousetrap and with a love of hunting the high and remote places, my son Jamie and I talked about what would be the ultimate lightweight hunting rifle capable of flattening our largest deer up close and personal on out to the long shots sometimes required in our big country. Setting ourselves a maximum all up including scope rifle weight of 7 pounds meant we would have to save weight wherever we could if we still wanted to have a barrel heavy enough to ensure excellent accuracy. Jamie already had a short action Remington Titanium and this seemed the ideal starting point to meet our light and compact criteria. The calibre was a foregone conclusion as the 7mm still provides the best compromise of ballistics versus recoil in lightweight rifles for our big game animals. But what case design would give those 7mm bullets the most velocity out of the short action? We wanted to see if we could come up with something that would get close to 7mm STW ballistics.

This full length cartridge is a superb long-range performer and is ballistically about halfway between the 7mm Magnum and my 7mm/404 wildcats or the 7mm RUM. The 7mm STW achieves 3200 fps with the 160 grain bullets we wanted to use, whereas the current short action velocity king, the 7mm WSM, produces just over 3000 fps with that bullet weight. We were going to need a lot more than WSM case capacity to achieve our objectives. Increasing case length wasn't an option as the WSM is already too long for an unmodified 700 short action, requiring magazine and bolt throw modifications to seat the bullets out to the lands for best accuracy. That only left using a fatter case, but the WSM's .550 inch base diameter was going to take some beating. Width wise the next step up the ladder was the 1911 vintage 416 Rigby and its more recent derivatives the 378 Weatherby and the 338 Lapua Magnum with their .585 - .590 inch base diameter. After some calculations it looked like a 35° shouldered version with a body length the same as the 7mm WSM would give us about the same capacity as the STW, leaving the question would the Remington 700 action handle the increased bolt thrust that a large diameter case like this generates?

Comparing calculations of locking lug surface area and shear strength, the Remington came out slightly ahead of the Mark V Weatherby action which has been offered in the 378 Weatherby and related chamberings since 1955. We decided therefore that this would be okay although we would keep tabs on locking lug setback by monitoring any changes in headspace. The other issue with a case of this diameter is the size of the barrel shank. At peak chamber pressure, the barrel shank expands and then returns to its original size due to not exceeding the steel's yield strength and elasticity. The brass cartridge case also expands but does not quite return to its original size due to its yield strength (about 7000 psi) having been exceeded, slightly wedging it in the chamber. This varies depending on how many times a case has been fired as the brass slowly loses its elasticity with repeated firings and sizings, increasing the wedging. When you lift the bolt, the leverage exerted by the primary extraction allows you to move the fired case rearwards enough to loosen it in the chamber due to the tapered chamber walls. The thinner the barrel shank around the chamber, the more it will expand before returning to size. The cartridge case therefore expands more as well, the wedging increases and the further rearward you will need to extract the case before it comes free. If the rearward movement required becomes more than the primary extraction provided by the action, you’ll have difficulty pulling back the bolt after you have opened it. The Remington 700 has a 1.0625 inch barrel shank the same as the Weatherby Mark V, but being a two lug action it has a 90° bolt lift which gives more primary extraction than the three sets of lugs and 60° bolt lift of the Mark V. As there haven't been any significant problems with the Mark V, we decided our extraction would be okay.

7mm Fatso
Most of the equipment necessary to form Fatso cases. The short starter and long expander mandrels can be seen in behind; the Neil Jones form die on the right; the eight bushings required for necking down the expanded and thinned case are middle left, the two neck sizing and shoulder bump bushings are middle right; the neck expander for neck turning is front left and the neck inside reamer is front right ; the blue neck turner is at the back ; 338 Lapua Magnum case at right ; shortened, expanded and thinned case in middle; 7mm Fatso loaded round with 160 grain Nosler Accubond on left.

Happy now that our action would handle the 416 Rigby family's base diameter, we looked at which of the family members we would use as our parent case. The obvious choice was the 338 Lapua Magnum due to the superb Lapua brass. The other options were the 416 Rigby itself or the big Weatherby, with Norma making the brass for both of these. The Norma brass although very accurate tends to be softer in the head area and the case life is shorter when loaded to the same pressure. The Lapua brass is also superbly accurate and is much harder in the head area, the only disadvantage being it has a very thick web which reduces case capacity somewhat. Settling on the Lapua, we finalised our reamer dimensions, using the same body length and shoulder angle as the 7mm WSM but extending the neck out to .350 inch as we had plenty of brass to play with. I prefer a long neck to keep the burning powder granules and gases deflecting off the shoulder further away from that most important throat area. Dave Manson of Manson reamers did his usual superb job of producing a quality set of floating pilot reamers and headspace gauges and had them out to me inside a month. Dave is the expert when it comes to these large case wildcats. Another reamer manufacturer I tried previously used blanks that are too small for this job causing chattering problems when chambering. I got Neil Jones of Custom Products to make a large diameter case forming die with eight bushings to progressively form the cases. Due to the large case diameter, we opted to use a 1 inch blank for the die body which meant making a 1 1/4" to 1" threaded bush to replace the usual 7/8" bush in the loading press.

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