OzFclass

Hi All,
If one were to consider a 7mm wildcat cartridge with no restriction to neck length, what would be the ideal length?
I'm guessing to have the base of the projectile at the neck/ shoulder junction to be clear of any possible donuts and be able to fill the case then how much to hold the bullet?
I have a 284 and a 280 AI which have rather different neck lengths.

Tim, I won't say much (lol). http://www.go2gbo.com/forums/wildcat-ca ... ngth-myth/. I have my own views of course.

Dave,
That was an interesting read.
Your 10c or $10 is also welcome

I think you need to first decide on a few bullets that you would ideally like to use. either weight, brand or ogive profile.

then determine the seating depth difference relative to the lands . eg a lighter bullet is going to be say .030" further out than a heavier bullet. that .030" is your "range" (may be more depending on the various bullet weights or styles your trying to include) so say you want to keep .020" from the neck shoulder junction, add your "range" .030" and then add the minimum neck engagement you'd like (say 1/2 caliber?)

.020+.030+.142= .192" would be your minimum neck length as an example.

I think your neck length is only really becomes critical "if" you have your throat length and leade angle optimized and you still can't seat the range of bullets you want with sufficient neck engagement and to also have the pressure ring clear the neck shoulder junction

Tim, the 7 mm is an excellent choice in terms of external ballistics. So let’s address the neck issue as it applies to many calibres as an internal ballistic discussion. Shooters have to see it in the context of the following. I know its long winded, but these are the linkages to understand.

Firstly, the throat length and leed (I use the original British spelling) angle are important as Adam has said, because they affect the barrel tune. The throat length affects the burn rate of the powder as it effectively lengthens the pressure vessel which is the chamber. This changes the pressures and finding a suitable pressure finds a suitable tune. The leed angle assists the transition of the projectile into the lands but it also has a significant effect on pressure depending on that angle and noticeably influences barrel lift. Longer throats tend to absorb pressure spikes due to primers but also projectiles of greater mass. So Adam’s point about knowing what to shoot is important.

However, the throat is a transient issue because it wears, and over a short period, that cherry ripe leed angle, that gives the perfect flip of the barrel for tuning purposes erodes and early testing nodes disappear. That’s why shooters at the top level go in with a fresh throat with a single competition in mind. It is this aspect that is important for team shooting.

Most people don’t have that luxury to nip up the chamber (importantly not altering headspace) and consequently the throat. So choosing a leed angle that is a closer match to the projectile ogive can mitigate against an abrupt change in tune we can sometimes experience and see as vertical shots which relate to barrel flip. That angle, is a tangent to the ogive at the desired touch point of the jacket with the barrel. This gives a more predicable tune and wears to ogive shape smoothly because of a greater surface area in contact. This applies to both jump and jam loads. That is not to say a jam load does not have other benefits in relation to concentricity and desired pressure. But the jam must be maintained. It too wears and still drops you off tune faster than a jump tune.

People may perceive fast throat wear when in fact, it is a smaller contact area that wears quickly. When the bullet shape establishes itself in the leed, somehow the leed wear miraculously slows down. Perhaps in the extreme, a large included angle in the leed, say 2 ½ degrees compared to 1 ½ degrees could give more resistance to wear and so hold tune longer. It would certainly increase pressure, more barrel lift in those otherwise neutral barrels and give a more complete burn of powder and more so if the bullet is jammed. So what we are looking at is a trade off between a reasonable tune window length and a very superior tune that will be of shorter duration.

The next issue to consider is concentricity i.e. how the projectile lines up in the throat. A projectile that does not line up in the throat centre will contribute significantly to in bore yaw. The projectile is malleable and will maintain its offset in the bore and into its trajectory; and upsets harmonics as well, with a condition called bullet slap - like mechanics see in worn cylinders and call piston slap.

The diameter of the throat is very important. The tighter the throat, the more it reduces the concentricity problem. Cases with short necks such as the Wolfpup to be successful must have matched diameters of the bullet to that of the diameter of the throat. It is like matching fitted necks to a chamber. Without the correct knowledge catastrophic failure is a distinct possibility. The Wolfpup with virtually no neck about .080" does however assist concentricity because of the tighter throat and because the neck length does not interfere with setup, the case is centralized by virtue of the case shoulder. Getting the headspace right is critical to alignment here.

It does make me think the throat diameter is more important not neck diameter and neck thickness, if you consider necks may be turned for fitting or people use no neck chambers with similar results.

As a fitting issue, the chamber cuts larger than the indicated dimensions on the reamer. So your throat may be looser than you think and not be an optimum match with your projectile. BUT, the gunsmith who does not have a matching reamer to groove size of the barrel can leave the slightest step in the throat. This happens when the reamer at the leed parallel section is undersized. It is a more common problem than you think. Ideally, the throat should be matched with a smooth transition to the groove and the touch point with consideration of the land height. A top gunsmith does more than you actually realise.

A longer neck length has the most benefit in my opinion on balance, but not from the view of concentricity if your reloading skills are up to scratch or if you have tighter throat which can compensate for poor reloading, but because it regulates gas flow.

As I have said before, the Ackley improved cases as well as minimising case stretch (which assists consistency in future reloading and allows for hotter charges), influences the turbulence of the gas produced from burning powder in a positive way by directing the stream of gas more efficiently. Modern cases like the BR neck angle may have refined this too. A longer neck also forces the gas to take a more parallel path and is less inclined to upset a boat tail projectile to cause in bore yaw which in turn gives you variances in trajectory and groups size due to unwanted vibrations affecting the harmonics as well.

Coming back to Adam’s point about throat length, the long neck enhances barrel life significantly and protects the bore from those hot eroding gases. The proof of which Xcalibre, on that other forum I linked earlier, demonstrates. So the longer neck, means the throat wear is minimised and protects that leed angle.

Depending on the chambering you choose, the longest neck is limited by the chamber. Don’t make the mistake of failing to trim cases either. The maximum case length should be .005” less than your chamber length for safety. You can buy chamber length gauges from Brownells. These are not headspace gauges but a plug you fit in a case that butts against the end of the chamber. It is used in the same way a dummy bullet is used to find touch length on the lands. David.

Another 2c worth.

For 7mm's its been proven that the 284W 284S and 7SAUM are all capable of winning consistently. I doubt the need to dabble with wildcats to make something better is actually there. Theres also the 7WSM, 280AI, 7-08AI to play with as well. I know of another form of 7mm magnum which looks to do well, but without custom dies and reamers etc is possibly not in everyones realistic capability to run.

Choose a proven case, practice shooting it in the wind and leave the technical stuff is how Ive been thinking over the last few months.

JMO.

Thanks to all for your input.
Brad, you just about quoted Matt Paroz word for word in regard to existing cartridges and their capabilities but he did add that sometimes you might have an itch you need to scratch
This is part of the fun of shooting in the open class

Tim N wrote:Thanks to all for your input.
Brad, you just about quoted Matt Paroz word for word in regard to existing cartridges and their capabilities but he did add that sometimes you might have an itch you need to scratch
This is part of the fun of shooting in the open class

So true Tim. If you have the itch, go for it. Been there and done it. Good fun
A thing to keep in mind, this "itch" has resulted in the Dasher, Shehane, Ackley Improved, so on and so on.

Indeed Tim, there are itches. Always a new case to look at. Toyed with the idea of fast twist 6PPC's with 95gr bullets for short ranges, short WSM's, necked down 9.3x62's, necked up 22-250's.

yep totally agree! what's out there can do the job for what we require BUT it doesn't mean it's the BEST it can be.

I think for our game there is a balance that you need to find between accuracy and reliability. yes optimizing throat lengths and angles etc may give you .05moa better accuracy but in 500 rounds through a SAUM I can guarantee all that optimizing is gonna be vaporized and spat somewhere 100yds down range

I personally would take a chamber, barrel, tune that is tolerant to weather, wear, fouling over one that is laser accurate one day and unpredictable the next.

I think the bigger advancement you may be able to achieve is being able to make a cartridge that is more tolerant and that is able to protect the throat and help maintain the critical dimensions for a longer period of time.