7mm/303 improved

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350velocette

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Anybody here re-load for a 7mm/303 improved.
Nearest I have found is for a standard 270/303 using ADI powders in one of their older loading books.
It's for my new/secondhand flat spring High Wall which I haven't shot yet as been in lock down for the last five weeks because of Covid19.
Be nice to find somebody already shooting this calibre and make it easy for me.
 
Welcome to this site. Hopefully we can help you.

I suspect that you are well into "wildcatter" territory with your chambering. Maybe you will be lucky enough to find pressure tested data. Kinda of need to establish the rifling twist in your barrel - that would suggest or limit 7mm bullet lengths - then need to investigate what weights of different brands of bullets fit that requirement. Then the throat length might limit or set depth of seating - would lead you to be able to establish powder volume with that bullet seated to that depth in your cases. Might then be helpful to work through reloading manuals finding pressure tested loads of cartridges in 7mm with similar weight bullets and similar powder volume - Nosler, for example, lists weight of water volume for every bullet seated to specified length for each cartridge. No clue where or how you are going to find reloading dies. Would hope original maker had thought of that - perhaps supplied with the rifle?? Most North American home made chambers are not required to be proof tested like in Europe, so likely out there on your own!

Or ignore all above - throw in some IMR 4350 and use standard large rifle primers - "work up" until you see "pressure signs" (whatever that might be in that cartridge in that rifle) and then back off a bit. In my mind, just so many things can go sideways without knowing the story behind that cartridge - especially the pressure tested data that developed it.
 
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RCBS dies came with the rifle and a few fired cases, 24 inch barrel and a one in ten twist.
Average weight of water in three cases comes out at 61.46 grains.
Last owner said the previous owner told him it had come in from Canada.
Regards.
 
Forty years ago I used to own a High Wall in 6mm/303 improved and used 42.5 grains of AR 2209 with a 85 grain Sierra open tip Boat Tail.
 
You're not going to find any pressure-tested loads for a "7mm/303 Improved" (which Improved?) cartridge in a "High-Wall", but some general guidleines below might assist.

You have a bore size (7mm) that's common, and quite a few 7mm cartridges for which load data exists, so for "Wildcats", using that data all depends on the case capacity of your cartridge in comparison to those common cartridges, and the action in which it's chambered.

Your case capacity will range from just above 7mm-08 to just less than 280 Rem.

If it's chambered in a strong action like P14 or a Ruger, then you could use 7mm-08 loads as a starting point and work up from there. As for how far above, inexperienced reloaders would be wise to stick with 7mm-08 loads.

If it's chambered in a "weak" action like a Lee Enfield, then you'll need to remain well below 303 Brit loads and certainly well below 7mm-08. As for 7X57, it has greater case capacity than the 7mm-08, so avoid load data for above 50K psi, but if you can find 7X57 loads for an action of in-between strength (e.g. 45K CUP), they could be useful as Eagleye has stated, with 7X57 Starting Loads as Max Loads. For a weak action, (or for a very cautious approach for any action) you could split the difference between 7-30 Waters and 7X57 loads.

I can't say if your action is "strong", "weak" or in-between, but if in-between as I suspect it is, then 7mm-08 Starting Loads as Max Loads would seem suited for it.

The problem with "looking for pressure signs" is that they are subjective, and thus notoriously unreliable for estimating pressure. Worst yet, in a "weak" action, the first pressure sign you might notice is a damaged firearm (but pristine primer pockets in the brass).



Here's an example:

150 gr bullet H4895 powder:

7-30 Waters - Max load 32.0 grs - 2350 fps
7X57 - Max Load 36.0 grs - 2380 fps

With a 150 gr bullet, Use 32.0-36.0 grs of H4895, or 2350 fps - whichever comes first.

A better measure of pressure for the Average Joe is a chronograph, and the knowledge of how to use it, but most don't own one.
 
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There is good advice above! Original 7x57 typically had 8.66" twist and was designed around 173 Round Nose bullets. Your barrel twist is slower, so likely want to start with bullets that are shorter (rifling twist rate and bullet stability is related primarily to bullet length, not bullet weight). I have some older Remington brand 175 grain Round Nose labelled "7mm Mauser 175 Grain Soft Point" which measure about 1.190" long. Some similar 7mm Hornady 175 grain round nose measure about 1.160. So probably want to start with bullets shorter than that. From an earlier discussion on CGN, for some reason Husqvarna produced commercial sporter 7x57 during 1950's with 12" twist. Discussion seems to be that they were designed around 150 grain Round Nose bullets, no doubt flat base, at that time.

If you are going to use the Nosler water volume data as comparison, understand they are reporting the weight of water in the case with that bullet seated - not to the brim of the case. They are trying to report the actual volume of space available for powder, once that bullet is seated to that particular length. Typically your depth of seating will be related to the length of the throat in your particular chamber - since your rifle is a "High Wall", there will be no magazine length to worry about.
 
As an example about the water volume thing, Nosler Manual 7 reports 61.5 grains of water in a 280 Remington Nosler brand case, with a 160 grain Partition seated to 3.330" overall cartridge length. You are reporting that same 61.5 grains of water, but I suspect that you mean filled to the brim So the volume available in your case will be considerably less than a 280 Remington. A quick review shows Nosler published pressure tested loadings with various powders, so that case volume thing is likely a good indicator that only way to achieve or exceed 280 Remington performance is to go to high pressures than they did. With that same 160 grain weight Partition they report 50.6 grains water seated at 3.045" C.O.A.L. in 7x57; 47.6 grains water in 7mm-08 seated to 2.800" C.O.A.L. You will have to research, but pressure limits are much lower for 7x57 than for the other two.
 
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Many thanks for all the advice, I live in NZ and we have a total lock down except for essential services, it's so quiet, only a few cases of Covid 19 compared to other countries, so it will be some time before I get this rifle shooting.
Yes the water was filled to the brim.
The 7mm I have only just bought and the 6mm was done by the late John Pell in Trinidad Colorado.
He had just finished a barrel for Hal Hartley and had the reamers so I went with that.
A few years later he also made me a a full length 22/303 and he also got me the action which came from a priest in New York of all things.
John was a real nice guy and flew as a gunner in a Flying Fortress over Europe and later qualified as a pilot and flew jets in Vietnam.
It's strange that a 7mm on a 303 case seems so rare.
In Australia they seem to go for a 22, 25 or a 270 with a 303 case and South Africa they went commercially with the 6mm/303.
Which is why I turned to Canada to see if not so rare there.
 
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Finding a starting load based on cartridge case volume is not that hard.
1:10 twist is common in modern 7x57 which will be pretty close to your case capacity.
I loaded mild 7x57 loads for my wife with 139 grain Hornady Interloc bullets. Of course, choice of bullet depends on your application.
Using normal start low and work up principle applies.
 
HERE IS DATA ON THE 270-303. YOU CAN USE THIS DATA AS START LOADS FOR YOUR 7MM IMPROVED.

Note - 2208 is sold here as Varget.

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Thank you for taking the trouble to copy and post the above chart but regarding the loads for a 150 grain bullet using 36.5 grains AR 2208 in the three ADI loading books I have from 2016 back to July 1996 the load listed there for a 150 grain bullet is listed as a MAX of 33.5 grains.
I am assuming all loads listed are for just a necked down version.
All sorts of difficulties arise when delving into the black art of wildcat cartridges.
Still if it was easy it wouldn't be fun.
Still on lockdown but going down a level after Tuesday. Still be a few more weeks until I can get to a range though.
At the moment still digesting what to put in the case to start off after all the advice and books I have looked at and still nobody who has actually loaded for this round.
 
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