7mm Projectiles - What's your limit?

[aaronraad]

So how much is too much weight - 185gr, 190gr, 200gr, 210gr?

How much weight is practical to use in 7mm regardless of cartridge chambering?

[bruce moulds]

possibly the question should refer to length and purpose.
what little i understand about ballistics.would cause the subject of precession and nutation to be raised at mention of a bullet longer than the berger or similar 180gn.
if a round nose, a heavier bullet can be had for the same length.
there have been attempts made to make bigh b.c. brass bullets. their extra length meant that no barrels in current use would stablize these bullets & they failed.
for hunting, even the biggest game does not require a bullet with an s.d. of greater than 3. a bullet longer than this will potentially bend on impact causing lack of straightness and depth of penetration.
more questions than answers.
keep safe,
bruce.

[Norm]

Aaron,
With a barrel 1 : 9 twist the 180gn range is about as heavy as you can go for a long pointy bullet. If it was a bluny nosed bullet you could go heavier.
So the limit is more to do with the fact that most target barrels are 1 : 9 twist and most hunting barrels run at 9.25 to 9.5 twist so no good there.

However Berger is producing a EOL 7mm 195gn pill that is interesting.
It has an estimated averaged G1 BC of .794 and a G7 BC of .406.

The bullet is 1.637" long will need at least a 1:8.5" twist to be stabilized.

So what you need is a bunch of shooters with faster twist rifles and they are not common but I will have one soon!

[AlanF]

So how much is too much weight - 185gr, 190gr, 200gr, 210gr?

How much weight is practical to use in 7mm regardless of cartridge chambering?

Aaron,

Here's some sectional densities of commonly used projectile weights in several calibres. If you look at the general pattern of increasing SD as the calibre rises, then maybe 7mm could go a bit heavier than 180, but not much. Something around 185 to 190 would be my pick, and you'd probably need to go with a magnum case head.



Alan

[Norm]

Alan,
Not sure that looking at the sectional density is of much use as it is a bit unreliable when trying to compare one calibre to another due to mass increasing by a greater proportion than cross sectional area.
For instance a 90gn .224 cal projectile has a SD of .256 and needs a very fast twist of 1 : 7 to be stabilised.
Compared to a .308 cal bullet with about the same SD .253 we have a bullet weighing 168 grains and it only needs a mild twist of 1 : 13 to be stabilised.

From a practical point of view I think that once the twist rate gets below 1 : 8 with a large calibre, heavy bullet you are pushing things. I.e. fouling, pressure and accuracy issues.
One possible solution might be to use a gain twist barrel? But I’m not sure how accurate they are.

[DaveMc]

Norm - I agree and think the twist rate is very important but not sure 7mm or 7.62 even work well at 8 twist. The gyroscopical forces (not stability factor) of a larger diameter bullet spinning that fast increase dramatically.

Now look at the rough practical limit that seems to be emerging from development over time. Of course this doesn't mean the boundaries can't be pushed but what happens is a smaller and smaller percentage of barrels/projectile batches hold together for consistent accuracy as you spin them faster. So we (driven by practical experience) move back to the "safe" area. There used to be a few 7mm 8 twist barrels around but you see very few of them now. If an 8 twist can't shoot a 180 grain consistently it sure as taxes will struggle with a 190+.

223 7 twist
243 7.5 twist
264 8 twist
284 8.5 twist
308 10 twist (maybe faster).

So I would look at what can stabilize in an 8.5 twist and go from there. There is some conjecture (very unproven) that the destabilizing lift forces associated with some of the newer shaped bullets are increased and mean they don't quite conform to the miller formulas and they perform better spun a little faster so we may quickly hit the wall down that approach. I look forward to you "thinking outside the box" on this Aaron.

[aaronraad]

Yes sorry, it was pretty much a question of how much you are prepared to 'twist & shout'. Obviously I'm no rocket scientist, come ballistics engineer, so I'm not expecting anyone to believe in my design concepts, but I've read my fair share of reports authored by more than one Ph.D. It is a forum after all not Projectile Aerodynamics Monthly.

An addtional 5gr at the top end of the scale only represents a small increase in 'wind bucking' ability. The same 5gr's though will also represent a decrease in accuracy as the length increases one way or another. How much accuracy is lost, I don't know?

So AlanF isn't that far from the mark in terms of an approach. I've attempted to address SD and failed twice. I've got one card left up my sleeve (unless depleted uranium is legalised), but I'll find out soon enough.

The next approach I'm taking is to design a more dynamically stable projectile that requires less twist for it's length etc. The first design component is almost complete but complicated. The tooling company feels confident though that the design can be completed, all be it I'm yet to see the cost. The second design component is still stuck in my head because it complicates the production process and I need to ensure it doesn't reduce the final quality of the projectile. At least it won't stop me from trying the first design component.

But yes, DaveMc is about right with the maximum twist vs calibre concept, all be it using copper jackets. If you want to see the full circle on barrel twist and velocity, look at articles on the design and failure of the 6.8mm SPC cartridge in terms of its long range advantages.

So don't throw out your old 7mm barrel yet. I might a need a few of you guys to make sure the throat is long enough and screw them back in for one last string.[/i]

[Matt P]

What about a high BC flat base, along the lines of the 187 BIB ???
Matt P

[IanP]

Look here: http://www.bergerbullets.com/products/target-bullets/

Berger not only give bullet data including BC and bullet length but now also have a Twist Rate Calculator (Miller formula) all in the one place for you to use.

The Twist rate Calculator is at the top RHS of the webpage!

Ian

[aaronraad]

What about a high BC flat base, along the lines of the 187 BIB ???
Matt P

I've got 200 sitting at home waiting for someone to trial in a slow twist barrel.

[Matt P]

How slow ?????

[aaronraad]

How slow ?????

I was going to punch them out of my 308W 1:13" twist barrel, but I'm not flush with time at the moment. I know the 1000BR shooters are pushing them out of 300 Win Mag set-ups (+3200fps?) using a 1:17" barrel, and even set a WR using the BIB's.

I'll post the specs tonight for overall length, but they are a basic flat base 10-calibre radius tangent ogive with about a 0.062" meplat. I was looking to market them to shooter's wanting to try F-T/R with a scope on their TR rifle with the barrel twist to suit the 155gr's.

[aaronraad]

Averaging about 1.3025" and 187gr

[BATattack]

Just my opinion but I think you do have to consider the case that will drive such a projectile.

Ideally they should be able to be pushed around the 2850-3000fps mark.

I shoot 7saum and 180gr bergers because I can load from 2900fps through to 3050 and hit several nodes in between at a velocity that I feel comfortable. Chime in everyone that said that velocity never won any prizes!! I’m just trying to show that you don’t really want to be aiming for a 2700fps max pressure node and not have the accuracy you want then have to drop back to 2550fps.

I think I would probably reach my pressure limit around 2900fps with a 195gr projectile. If I can’t get an accuracy node at 2900 my only option is to load down . . . how low before I get another node is unknown.

If you go much heavier than 190gr you are going to be using somewhere around 68+gr of 2217 or similar. By using more than 68gr of powder you are going to be quickly reducing usable barrel life.

A case similar capacity to a 7WSM would be able to make use of a 200gr projectile but the WSM has gone out of favour due to the short barrel life.

Just my opinion but I wish you all the best and look forward to trying some of your hanywork down the track.

[aaronraad]

Just my opinion but I think you do have to consider the case that will drive such a projectile.

Ideally they should be able to be pushed around the 2850-3000fps mark.

I shoot 7saum and 180gr bergers because I can load from 2900fps through to 3050 and hit several nodes in between at a velocity that I feel comfortable. Chime in everyone that said that velocity never won any prizes!! I’m just trying to show that you don’t really want to be aiming for a 2700fps max pressure node and not have the accuracy you want then have to drop back to 2550fps.

I think I would probably reach my pressure limit around 2900fps with a 195gr projectile. If I can’t get an accuracy node at 2900 my only option is to load down . . . how low before I get another node is unknown.

If you go much heavier than 190gr you are going to be using somewhere around 68+gr of 2217 or similar. By using more than 68gr of powder you are going to be quickly reducing usable barrel life.

A case similar capacity to a 7WSM would be able to make use of a 200gr projectile but the WSM has gone out of favour due to the short barrel life.

Just my opinion but I wish you all the best and look forward to trying some of your hanywork down the track.

Couldn't agree more BATattack. If we limit ourselves to case design and barrel life though, then we'll be exactly where we were 100 years ago, re-inventing PO Ackley designs (whoops to late!). I'm just trying to lift the lid a little on projectile design and hopefully the rest of you guys will follow (insert irony here - see Barnes/Fowler). F-Class target rifle has one big advantage in terms of design - single shot only. Here is a hint, start thinking overall cartridge length.