I just started working up loads for my .20 VT. I started well below the max listing that I could find. I also loaded to an overall length that was suggested, but they looked pretty short. I started showing signs of pressure way before I reached anywhere near the max reccommended load. Further research found a much longer (.090 or more) recomendation. My question is .. for the same charge will bullets seated longer reduce pressure. Does a compressed load change pressure? Please give me some general advise. Thanks

Dale
When you push the bullet further into the case,you make a smaller"boiler room"andthus,increase the pressure.
Move the bullet out to decrease pressure,until you touch the lands in the barrel.That gives you a major increase in pressure.
Frank

Dale
When you push the bullet further into the case,you make a smaller"boiler room"andthus,increase the pressure.
Move the bullet out to decrease pressure,until you touch the lands in the barrel.That gives you a major increase in pressure.
Frank

Makes sense. I put together a few with the "safe' load but loaded longer. I'll test fire those and see from there. Thanks

Try here: http://kwk.us/powley.html

Be sure to read the notes link at the bottom of the page. You can play with all kinds of variables (case capacity, seating depth/bullet length, weight, etc.) except primers. The absolute accuracy may not be great, but you can get an idea of the trends.

Andy

The reality is that for a small bore diameter bullet in a large case, seating depth will have very littel or no effect. It is a cause for concern in straight wall cases, usually those with small case capacities such as 9mm, .38 Special, .44 Magnum. In those types of cases, a small difference in seating depth will have a significant effect on the case capacity and can quickly cause an overload.
Not in a .20.

Generaly for a bottle neck cartridge you start with .015-0.20 inch from the lands to determin your max pressure for that gun. Then you and usually at the range seat say 5 rounds 0.005 inch deaper and shoot them. Then 5 more 0.010 deaper untill you find the most accurate seating depth. The major resistance that will raise your pressure is the rifling or lands. So the further from the lands the less pressure because the bullet gets to move before contacting the lands and the more distance the less pressure. Read your reloading manual it will discribe this in more detail. All my Hornady, Nosler, and Speer manuals discus this in detail.
Is this the hornady .20cal ?

Seating the bullet out further lowers pressure by increasing powder space right up until the bullet touches the lands. At that point there is a jump in pressure. This is due to contact with the lands causing the bullet to start engraving from a standstill. In bottleneck rifle cases, it is typically equal to adding 0.5 to 1 grain of powder, depending on the cartridge dimensions. As long as it gets a running start, there will not be time for the bullet to form the static friction component. The result is a curve with a jump in pressure as shown in the example below.

http://img297.imageshack.us/img297/6446/223pressurevseatingdepzm6.gif

Seating the bullet out further lowers pressure by increasing powder space right up until the bullet touches the lands. At that point there is a jump in pressure. This is due to contact with the lands causing the bullet to start engraving from a standstill. In bottleneck rifle cases, it is typically equal to adding 0.5 to 1 grain of powder, depending on the cartridge dimensions. As long as it gets a running start, there will not be time for the bullet to form the static friction component. The result is a curve with a jump in pressure as shown in the example below.

http://img297.imageshack.us/img297/6446/223pressurevseatingdepzm6.gif

It looks like two different opinions. However, I can see that touching the lands would increase the pressure. Yes it is the Hornady .20 cal 32 gr. v-max. Thanks for the input.

It looks like two different opinions. . .

Not opinion; fact. If you are as addicted to the hobby as I am, and have a laptop, you can buy RSI's Pressure Trace (http://www.shootingsoftware.com/pressure.htm) instrument and measure it for yourself. In this case, to make hard numbers quickly and not have to sort statistical variance out, I ran the decreasing pressure/seating depth loads in the QuickLOAD (http://www.neconos.com/details3.htm) internal ballistics simulator. For the lands touching, I increased the start pressure number corresponding to engineering tables of the proportion of copper-on-steel static coefficient of friction to kinetic coefficient of friction (0.53:0.36). But it corresponds to what the pressure measuring gear shows very closely. The calculation shows touching has the equivalent of 0.4 grains of powder increase in that .223 with that bullet, and 0.5 with lighter bullets and Reloader 10X. Measurements I've made put it closer to 0.5 and 0.6 grains for both, but that's not too shabby as such estimates go. Other powders will have different energy densities, but 748 in the graph is pretty typical of the 3800-4000 J/gm range most rifle powders fall into. In .308 it comes out closer to 0.6 to .7 grains. If you assume you should decrease your just-short-of-touching load by 1 grain, and work back up from there, you should be pretty safe in developing a land touching load.

Hornady number 7 loading manual page 25, and call there technical help and disscus it with them. Try nosler and hodgden technical help too.

It has to do with the pressure curve and thats not static. When the bullet moves the expanding gases have room to expand and less pressure per time is the resault. The crimp is minimal resistant compared to the resistance of the rifling.

A train engine can not start moving the whole train at once. It moves one railcar 14 inches than the draftgear (linkage) bottom out its moumentum and that of the first car now pull the third car 14 inches and so on. By the time the last car gets pulled you have the moumentum of all but one railcar to perswade it to move.

How does this apply to your bullet? There is neck tention and or a crimp on the bullet the expanding gases start to rise and push the bullet from the case the increasing room keep the pressure from an intense high pressure spike when the bullet engages the rifling it has moumtum and keeps moving. Remember its a pressed fit in the barrel. Further back the more room and mountum. If it was just touching there would be no increasing room for the expanding gases and pressure can peak very fast and a safe shorter OCL powder chage can now be to much powder for the longer OCL and now the charge density and its effect on burn rate can come into play to effect the pressur cruve.

Please read your manuals and consult powder companys technical servises as well.

How does this apply to your bullet? . . .it has moumtum and keeps moving. . .

Unfortunately, this does not really explain it. If it did, the momentum would be greater as the bullet was seated deeper and had more distance to run up to higher momentum before touching the lands. This would reduce the start pressure and the resulting peak pressure would then go down as you seated the bullet deeper, but it doesn't. It goes up because the powder is making gas in a smaller space, thereby shortening the time to reach peak pressure. Also, it does not explain why a bullet as little as 0.001" off the lands, having almost no room to develop momentum, still does not develop the pressure jump that begins when a bullet touches the lands.

The actual cause is the difference between static and kinetic coefficients of friction at the throat where the bullet first touches the lands. This difference exists between all materials, though the ratios of the two vary greatly and depend on the materials, their surface texture, and sliding velocity. The explanation in the Wikipedia (http://en.wikipedia.org/wiki/Coefficient_of_friction) is adequate and minimally technical. The important point from the article is this:

. . . The coefficient of static friction is larger than the coefficient of kinetic friction: it takes more force to make surfaces start sliding over each other than it does to keep them sliding once started. . .

. . . The friction force is electromagnetic in origin: atoms of one surface "stick" to atoms of the other briefly before snapping apart, causing atomic vibrations, and thus transforming the work needed to maintain the sliding into heat. . .

When I was in school, the best explanation of the difference was that pausing one surface over the other allowed time for stresses introduced by the forces normal to the contact to relax and let the surfaces flow into greater contact microscopically. This allowed more area for electric bonds to form than when contact is less complete. That was 35 years ago, and I don’t know if the thinking has changed any since? The experimentally observable difference remains. If you set a flat object onto a plate, it takes more force to start it moving than it takes to keep it moving. How much more is proportional to the difference in the static and kinetic coefficients of friction for the two materials and surface forms. For a table of the two values for many common pairings of materials, click here (http://www.school-for-champions.com/science/frictioncoeff.htm).

I have no need to argue or promote my selfe thats why I stated to consult the verious powder and bullet companys. If you differ with thier published data please take that it up with them. I do have at my disposal PHD's in both metalergy and physics it comes in great at times for coffee pot discussions.

Personal I seen both resualts with just vering the OCL as measured from the lands by .015-.025" just to many veribles but none where dramatic differances. This is mainly a lamens forum and keeping the discriptions and terms at a lamens level is help full to all most wont care about the metal grainular interactions or its coefficient of friction just its resualting effect on their desirerd outcome.

This is not a challeges just time to reflect on ones own posts sometimes the reader may not get the writters indended message.

I have no need to argue. . .

Not meaning to be argumentative. Sorry if it reads that way. I am just explaining why the momentum theory doesn't match observed pressure behavior, and where to look up the coefficient of friction information which does. Your advice to talk to the manufacturing techs is always good to do.

You are correct that most people reading the site are casual laymen, and will probably doze off before they get to the bottom of my post. :o. There are some of us, though, who are interested in the technical details. I'll let people who are disinterested skip the tech talk by their own choice, but I won't dumb it down to hold their interest. Like chosing to change a TV channel or fast-forward a VCR over the commercials, you always have the option to skip it.