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270 and 7mm Wildcats Sub sonic

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Unread 11-09-2006, 09:00 PM
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270 and 7mm Wildcats Sub sonic

Hi all i have been thinking of making a sub sonic rifle for a few years and was looking at a 338 and then i had a thought why not use the 6.8SPC case it would push a 190+ gr .277 or 200gr .284 projectile over 1000fps with ease. I can get a 270 or 7mm blank in 8 twist so either should work i assume?

The advantage with the 7mm over the 270 is that their are more heavy match projectiles available like the 180gr Berger.

Wich way would you go? 7mm is my thought with a 17" barrel and mabe carbon wrapped. but the 6.8 is a factory chambering so their are advantages there aswell.

Cheers Bill
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Unread 11-09-2006, 09:10 PM
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Re: 270 and 7mm Wildcats Sub sonic

I would do a 338 on the 6mm br case. the 300 grain smk would have more energy than a 200 grain wildcat and you can get a 8 twist 338 barrel as well. I have been pondering the idea for some time as well

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Unread 11-09-2006, 09:41 PM
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Re: 270 and 7mm Wildcats Sub sonic

if i do go 338 i will still use the 6.8 case i have some made up and shortened it lookis awsome and will feed through an AR mag that is used for the 6.8 and it ejects correctly from a rem 700 where the BR cases are painfull to eject.

Cheers Bill
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Unread 11-10-2006, 05:48 AM
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Re: 270 and 7mm Wildcats Sub sonic

i have a 338 subsonic gun its base off br case works great. feeds ok nothing great but its come out prefect not one catch. would not do anything smaller else it just for paper shooting and close at that. if you want more answers just ask ill post later.
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Unread 11-10-2006, 07:41 AM
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Re: 270 and 7mm Wildcats Sub sonic

The only reason i will not do the 6.8 spc case is because the bastard head size. in a ar platform that would be ok, bolt heads are availbale. In a bolt gun you have to either open up a .378 bolt head or buy a dedicated 6.8 spc gun.

since you are looking at an AR platform why not do it of the ppc case? you can get the 6.5 grendal cases that will feed through a slightly modified 6.8 mag, bolt heads are available, and you could always get you a 6mm AR barrel for it as well. One gun for two purposes.

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Unread 11-10-2006, 09:34 AM
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Re: 270 and 7mm Wildcats Sub sonic

Since twist is velocity dependant, wouldn't you need more twist at such low velocities? I thought that high B.C. bullets generally need a good velocity from a marginal twist.

I am not stating, I am asking.

Inquiring minds [img]/ubbthreads/images/graemlins/smile.gif[/img]

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Unread 11-10-2006, 10:19 AM
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Re: 270 and 7mm Wildcats Sub sonic

While it is certainly true that the 338 will have more retained energy it may not be as much as one would thing. If we look at some general numbers comparing the 300 gr SMK and the 7mm 200gr ULD RBBT, both loaded to 1040 fps which is pretty standard for a whisper type of round. There are some interesting things that pop out at you.

First off energy, yes, the 338 has an advantage. Here are the numbers:

300 gr SMK.........720 ft/lbs
200 gr ULD RBBT....480 ft/lbs

Big advantage for the 338 right? well lets look at what happens as the range increases:

500 yards
300 SMK............570 ft/lbs
200 gr ULD RBBT....391 ft/lbs

What was a 240 ft/lb advantage is now a 179 ft/lb advantage. At longer range the gap tightens even more.

1000 yards
300 gr SMK........467 ft/lbs
200 gr ULD RBBT...328 ft/lbs

Now the advantage is down to 139 ft/lbs of energy. Still a significant amount more when considering we are comparing energy levels under 500 ft/lbs.

If this were the only criteria to look at the 338 would be my choice but it is one of many that we look at for long range performance.

Personally, more important then energy is drop and wind drift. Drop is not terribly important because its easy to figure and is a relative constant. Wind drift is not.

If we look at midrange height and bullet drop at 1000 yards with both rounds zeroed at 500 yards:

Midrange height
300 gr SMK..............+112" @ 260 yards
200 gr ULD RBBT.........+109" @ 255 yards

Not much of a difference here. At 1000 yards though bullet drop difference will be a wider spread:

1000 yard drop
300 gr SMK............-1012"
200 gr ULD RBBT.......-984"

Thats a bit more then 2 feet less drop, again not dramatic but the lighter 7mm has the edge here.

If we look at wind drift we again see the 300 gr bullet really has no advantage:

500 yards
300 gr SMK.............16.1"
200 gr ULD RBBT........14.1"

1000 yards
300 gr SMK.............62.6"
200 gr ULD RBBT........54.6"

Here again, the lighter bullet has the advantage but not a dramatic advantage but still its there.

As you can see, the 300 gr pills has slightly more energy but the 200 gr bullet has less drop and less drift and more importantly, less recoil. If we look at the recoil numbers in a 8 lb rifle for comparision both loaded to 1040 fps:

300 gr SMK..........8.9 ft/lbs
200 gr ULD RBBT.....4.8 ft/lbs

This is relatively significant. While neither recoil levels are anything near the level that would be uncomfortable in any way, there is a significant difference. The 200 gr ULD RBBT will recoil basically identically to the feel of a 22-250 loaded with a 55 gr bulllet at 3600 fps in an 8 lb rifle, really not much.

The 300 gr SMK will have the same recoil as a 243 Win with a 100 gr pill loaded to 3100 fps in an 8 lb rifle, again not much recoil.


Anyone that has shot alot of rounds through both rifles will tell you the 22-250 is MUCH better suited for long shooting sessions compared to the 243 which will get a bit twitch running alot of rounds though a single session.

SO while the 338 certainly has its advantages it is not all that superior to the 200 gr 7mm ULD RBBT and in fact in most catagories it falls short to the 7mm.

I would not recommend the 270 personally. The 195 gr pill is a great bullet and actually seems to be producing a BC a slight bit higher then the 200 gr ULD RBBT 7mm bullet but there are no other heavy options in this caliber range. I would personally go with the 200 gr 7mm and to be honest I may look at a 1-7 twist from Rock with a thinned land design which they have made for the 200 gr ULD RBBT.

There are plans to offer even heavier 7mm bullets up to the 250 gr class range as well and the 1-7 would let you shoot all of these at these velocities.

Just to throw another iron in the fire, Richard is now making a 300 gr ULD FB bullet in 30 cal as well!!!

Good Shooting!!

Kirby Allen(50)
Kirby Allen(50)

Allen Precision Shooting
Home of the Allen Magnum, Allen Xpress and Allen Tactical Wildcats and the Painkiller Muzzle brakes.

Farther, Faster and Flatter then ever before.

Web Page: www.apsrifles.com

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