The 7mm Fatso - The Ultimate Short Magnum!
While waiting for a new order of barrels to arrive from Dan Lilja in Montana, we opened up the 7mm SAUM chamber on the factory Titanium barrel with the new reamers to make sure the concept was going to work. We also opened up the bolt face and fitted a Sako style extractor designed to work with the 338 LM. We extended rearward the magazine cut out in the bottom of the receiver, shortened the bolt stop to lengthen bolt throw and fitted a Wyatt extended mag box. We opened up the feed rails slightly to allow the fatter case through but this was the only feeding modifications needed as the Wyatt Mag box has its own centre feed lips. A one-piece Weaver aluminum base was epoxied on, Nightforce aluminum/titanium 30 mm rings lapped and a 4.5-14x50 Mark 4 Leupold fitted.
We then formed up some cases which entailed the eight step neck and shoulder forming operation followed by trimming to length, an inside reaming and outside neck turning, and then fire forming. Extraction was difficult with heavy initial bolt lift, not due to the possible issues raised above, but caused by the hard and thick brass that was now formed into the new shoulder. Annealing alleviated the problem somewhat but we still had to excessively bump the shoulder back in a special die we made after each firing. This excessive sizing would eventually lead to case head separations, so we decided there had to be a better way. Thinning down the brass where the new shoulder was going to form seemed to be the only solution so we made up a set of inside expanders and turning mandrels. We set it up so we could take a simple parallel cut with the lathe reducing the case wall down to a thickness more suitable for the shoulder area. These thinned cases were then formed up as before and this solved any extraction problems with the fired cases just falling out.
Turning the expanded and shortened case to reduce the brass thickness where the new shoulder will form.
Initial load development proved interesting as the hard Lapua brass didn't show the usual pressure signs, so we were very conservative while working up loads. We chose the Hornady 162 grain A-Max as an accurate long-range bullet to begin with, and were pleased to achieve 3200 fps with the 24 inch factory barrel. This was with the new ADI 2225/Retumbo which proved to have about the perfect burning rate of any of the extruded powders in this case. It looked like we would equal 7mm STW velocities in the same length barrel which was our original objective. Surprisingly, the factory barrel held half MOA accuracy most of the time, a testament to the quality of the barrels Remington put on their Titanium’s – or this one at least!
After the initial range work was completed and drop chart generated, Jamie was busting a gut to kill something with the new round which at this stage we called the 7mm LSM (Lapua Short Magnum). It was time for our annual school holiday trip into the mountain beech area of the Kaweka Mountains. Jamie pulled off a magnificent 650 yard shot holding one mildot (approx 3.5 MOA) of windage to kill a Sika hind on his favourite clearing on a windy day; a great way to blood a new cartridge! The following day he also shot a dry hind at close range while stalking the bush edge, before an easterly storm finished our trip early.
The first game animal taken with the 7mm Fatso, original factory barrel. A sika hind shot at 650 yards in the mountain beech area of the Kawekas.
With the roar (rut) approaching and the new barrel still on the water, we thought we’d better develop a load for larger deer at close range. Of the controlled expansion type hunting bullets, the factory barrel seemed to like 160 grain Speer Grandslams best. A moderate load achieved 3150 fps with accuracy around one MOA giving suitable performance out to over 300 yards.
The first roar trip for the rifle was a wander around the bush out the back of my uncle's farm. While the population of red deer is fairly low, you can encounter the odd animal especially during the roar when the stags are advertising their presence. Early one Saturday morning, we had made our way to a good knob to listen from and were pleasantly surprised to get a reply to my first red roar for the year. Making our way through kanuka scrub to a gorgy area about where we thought the reply had come from, I gave another roar and spied a set of antlers moving in the scrub on the next ridge. Looking through our binos we could see he had a fair bit of antler up and was definitely worth shooting. Just then he moved off heading up the ridge and we thought he must have twigged us so we raced off on the parallel ridge trying to catch up. At a dip in the ridge we heard a crash on the right and it turned out he’d been making his way across the gully to us. Now we had spooked him, so I roared to try and stop him. This worked and he paused as he crested his own ridge 100 yards away. Jamie thought he could see enough of him and wasted no time in dropping down and shooting off his knees. The shot had the desired result, so we made our way over to find he'd secured an uneven 12 pointer, but much better than we'd expected from this area. He had a large body in very good condition so we got all of him out and he made wonderful sausages!
Jamie and an uneven 12 pointer, the first red stag to fall to the 7mm Fatso. Taken with a 160 grain Speer GrandSlam at 100 yards across a gully in Big Kanuka.
After that I was so busy guiding on sika that the boys had to make their own arrangements. On a fly camping trip into the Southern Kaimanawa Wilderness Area, they shot a lovely eight point sika – a combined effort with Willie hitting it first with a hurried shot with his 25/284 as the stag departed. Then Jamie followed him up on the run and finished the job with the Fatso.
The boys and their trophy 8 point sika stag
My good friend and accountant Ian Lyver was doing a May trip into the wapiti country after the bugle with his son Phil, and Jamie loaned them his rifle. After making their way up to the Edith saddle, Ian and Phil spent a few days hunting in the vicinity. One day while making their way up the large terraces that lead to Oilskin Pass, they encountered a pretty good 13 point crossbreed chasing some cows. The wapiti had got their wind and were departing but the bull paused on the edge of a ledge over 300 yards away. Ian dropped him only to have him fall over a bluff and smash the bey tines right off. Such is luck!
Ian Lyver with his wapiti with the Edith Valley in the background. The bull fell over a bluff after the shot and smashed tines and broke his nose.
Finally the new barrel order arrived, including a 26 inch fluted number 4 contour stainless match earmarked for Jamie's rifle. We set to and trued the Titanium action and then fitted and chambered the barrel. We fitted a 3 lever Jewel trigger and bedded the barreled action into one of McMillan Hunters Edge lightweight carbon fiber rifle stocks. Then a Nightforce NXS 5.5-22x56 scope was installed for testing purposes.
After the one shot and clean barrel break in (which took eight rounds for the barrel to stop copper fouling), the serious load development began. This time we used an Oehler 43 ballistic laboratory with a strain gauge fitted on top of the barrel above the chamber area to monitor pressure precisely. We started with the 162 grain A-Max that had performed well in the factory barrel and this time the top velocity producer was Hodgdon's H870 ball powder with maximum loads approaching 3300 fps. Next we tried Berger's 180 grain VLD which with the same powder achieved 3100 fps. Lastly we tested 160 grain Nosler Accubond's seated to fit in the magazine and pushed up to 3300 fps as an all-round big game load.
Accuracy wise the A-Max's and Berger's both averaged better than .4" at 100 yards, while the Accubond big game magazine loads managed better than .6" even though they were jumping a long way to the rifling. They also shot to the same point of impact as the others which was a lucky bonus.
Now it was down to the long range testing to see whether the A-Max or the Berger would be chosen for the distance work. They both shot extremely good groups at 600 and 1000 yards averaging half MOA with very little vertical in them. The BC’s worked out to be .650 for the A-Max and .685 for the Berger out of this barrel, which ballistically gave the edge to the A-Max due to the 200 fps extra velocity. This is the lowest BC I have ever got for the 180 Berger which is unusual as out of some of my barrels it works out at over .700, proving that all barrels can be a law unto themselves.
Lastly accuracy tests off a bipod were conducted which proved acceptable so long as you had a hand firmly upfront. Even with the big heavy Nightforce on top, the all up rifle weight was still under 8 pounds which made it quite lively on the bench especially when testing the heavyweight 180 grain VLD's. We decided to fit a Vais muzzle brake so turned a custom thread at muzzle diameter (.650”) on the barrel and threaded the muzzle brake to suit. This changed the personality of the rifle completely and you could now shoot it very accurately with your left-hand under the butt which I prefer in the field.
Interestingly, the point of impact hardly changed at all and you could easily interchange from the aluminium thread protector which is easier on the ears for walking and stalking, to the muzzle brake for long-range sniping when you've got time to use earplugs.
Use enough gun! Although a little unnecessary for wallabies, the Fatso made short work of these at 600 yards.
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