|
Post by Kevin B. on Feb 3, 2008 18:30:04 GMT -5
Lets say, for the purpose of this question, that my ML2 is loaded with 61 grains of N 120. Next is a Harvester short black sabot, then the Hornady #4500. Please try to visualize the barrel cut away to show this. In the primer pocket of the stock breech plug is the Fed 209A. Not my favorite, but I have plenty. The primer is dented and the primer ignites, shooting fire through the breech plug, thru the vent liner, which meters the amount of fire (I assume) which streams into the load of 120. Here is the question, and it is for anyone to answer. WHAT HAPPENS NEXT. Obviously, the powder burns. But in super slow motion, what does the saboted bullet do? Thanks ahead of time. And, it's not a trick question. I'll go first: The pressure rise immediately expands (flares) the sabot base acting as a gas tight seal. Inertia causes the bullet to begin deforming into the shape of least resistance and therefore creating yet more resistance. (obturation) Then, and only then, does the saboted bullet begin it's travel down the tube. If the bore is rough ( as the Savage ML bore is wont to be) then the plastic gasket will encounter an abrasive effect, which may or may not effect the way it acts as a carrier. Anyway, that's what I think is happening.
|
|
|
Post by encoreguy on Feb 3, 2008 20:20:44 GMT -5
Here is what I think happens, the primer actually pushes the sabot a little prior to powder ignition, powder starts to burn as sabot base expands and builds pressure. I honestly don't think the bullet expands much to increase pressure. I think it is just along for the ride in the sabot. The sabot does all the work and that is the problem with some or the inconsistency and why standard deviations can be rather high in the velocity category as compared to centerfire rounds.
|
|
|
Post by edge on Feb 3, 2008 21:17:25 GMT -5
Two years ago I would not have thought that the bullet could possibly expand inside of the bore. That was before I shot a buck and recovered the bullet. Now there is a fair difference between what I shoot and what you might shoot. First, the bullet was a 8mm 150 grain Sierra ProHunter from my modified Savage ML-II. Velocity was over 3,000 fps, and the sabots were rigid PVC. Below is a picture of an uncut sabot with pristine bullet and the one recovered. Notice the shape of the cutout in the sabot and the mirror shape of the bullet. Recovered bullet from the base. Note the slight protrusions where the bullet expanded into the saw cuts. The bump in the center is the mirror of the drilled sabot. Pristine bullet and recovered to show how much it expanded in the barrel ( notice the saw cut remnants). I changed the design after recovering this bullet, and switched to the solid base AccuBond. edge.
|
|
|
Post by tar12 on Feb 3, 2008 22:16:11 GMT -5
Edge, How thick are your sabots?
|
|
|
Post by DW on Feb 3, 2008 22:55:56 GMT -5
I don't know if this is proof of expansion, but I have looked at numerous spent sabots, and at the point where the base of the SST bullet sits in the sabot the petals flare out like the base of the bullet expanded. Now in the .45 when using the 195 BX, this is not as noticeable when examining the blue Harvester sabot, probably because of the all copper bullet.
|
|
|
Post by edge on Feb 4, 2008 6:49:56 GMT -5
Edge, How thick are your sabots? The one in the picture is for a 505 Gibbs barrel and an 8mm bullet which makes for a 0.091 max wall. The thin section where the bullet flowed tapers down to approximately 0.025 thick. edge. PS remember that this is a jacketed bullet, so those that use a Pure Lead bullet might want to remember this picture
|
|
|
Post by savagebrother on Feb 4, 2008 7:54:41 GMT -5
edge, white muzlleloaders have depended on bullet obturation for there accuracy and ease of loading for a long time. of course its easier to obturate a big lead conical than a jacketed one. sb
|
|
|
Post by edge on Feb 4, 2008 8:11:04 GMT -5
To clarify my point, IMO, if you buy a non jacketed pure lead bullet because the B.C. is much higher than a SW or other such jacketed bullet, then I think that the bullet exiting the bore may not be the same shape as when you loaded it!
I would be highly surprised to hear that a pure lead bullet were even round when fired from the Savage in a sabot! I bet that it is much shorter, and has "fins" where the lead squeezed into the slots of the sabot.
edge.
|
|
|
Post by rossman40 on Feb 4, 2008 19:12:53 GMT -5
Wikipedia was a nice article on orbturate,, en.wikipedia.org/wiki/ObturateWith the table they show pressures required to get a good seal,, Material BHN Pressure Pure lead 5 7,110 1:20 tin/lead 10 14,200 1:10 tin/lead 11.5 16,400 Pure copper 40 56,900 Thicker jackets would require more pressure and it goes a way to explain how Wilmsmeyer gets better groups with Barnes bullets by shooting those heavy loads.
|
|
|
Post by wilmsmeyer on Feb 4, 2008 20:02:36 GMT -5
Interesting and enlightening.
|
|
|
Post by jims on Feb 4, 2008 21:39:04 GMT -5
Now that it is all spelled out it makes sense.
|
|