Obturation

[lightload]

Do jacketed bullets obturate? For example, does a .355 jacketed bullet fired in a 9mm barrel that has a .358 groove diameter obturate?

[BigAlofPa.]

Yes they do. My Caniks i can't shoot .356 hard lead cast. BHN 18. Bore leads up. But i can shoot .358 in it. Now the jacketed and plated .355 i can fire in it and no leading or copper fouling. Hope this helps.

[ddixie884]

Some what............

[Hick]

Gonna go out on a limb here (if I'm wrong I'm sure someone will let me know). as-fired jacketed bullets I have seen still have the shape they had before firing. Boat tailed bullets are still boat-tailed after hitting a berm. I suspect, therefore, that any obturation in a jacketed bullet is minimal, of not zero. Undersized jacketed bullets (within reason) will shoot without bore fouling, but I don't think that proves obturation is happening. Could be wrong. but that's my take on it.

[BigAlofPa.]

I have some fired jacketed 9mm bullets i found on the range. I'll mic them then.

[country gent]

Not sure at handgun pressures / velocities if the jackets expand much put Im thinking a small amount at peak pressures. At modern rifle pressures and velocities Im pretty sure they do obturate some. this would be ahead of the base boat tail. Slamming the base of the thin jacketed lead cored bullet with 60,000 cup or more will have some effect. The reason for the boat tails retaining their shape is the 2 folds 1 bullet dia and the other smaller with the short tapered section is much stronger / stiffer than the cylindrical section ahead of the fold.

Another factor would be jacket thickness / hardness harder thicker jackets would resist this more. Sierra jackets are harder than J-4s On swaged 22 rim fire cases are normally annealed to be dead soft to aid forming and may also aid this.

One other thing to consider is on most firearms jacketed bullets are much closer to groove dia of the barrels meaning they have less to swell to fill the gap.

[DougGuy]

Jacketed bullets DO obturate. Pistol bullets are much more prone to this than rifle, I think as a general rule pistol bullets are made with thinner jackets because rifles operate at much higher pressures. So comparing a .308" bullet for a 30-30 you have a jacket that might be .007" or so thick, with a soft lead core swaged into it that's only .294" inside the jacket. Looking at the comparison, the jacket is 4.76% of the core, now lets compare to a .451" jhp where the jacket is only .0035" and the core is .444" the jacket is only 1.58% of the core, so there is much less jacket material to resist obturation. The core is much shorter in comparison to it's diameter than a rifle bullet, so, to answer the question in this manner is only fair since one needs to take into account whether they are determining the likelihood of obturation for a rifle bullet or a pistol bullet.

A j-word pistol bullet will obturate VERY easily compared to a rifle, generally the cores of pistol bullets are pure soft lead swaged into the gilding metal jacket, a whole lot softer than even 50/50+2%, as evidence of this, I offer the fact that I can hone cylinder throats quite a bit larger than the jacketed factory bullet, and all but the lowest pressure rounds will shoot pretty good and I never hear a complaint from the customer stating that cast shoots well but jacketed factory ammo shoots poorly. If anything, jacketed is improved because now the bullets are leaving the cylinder larger than they were when they were loaded, and they seal in the bore even better.


Do jacketed bullets obturate? For example, does a .355 jacketed bullet fired in a 9mm barrel that has a .358 groove diameter obturate? Considering the 9mm operating pressure is around 35kpsi max, I would say that this is one caliber that obturation is pretty near guaranteed.

[BigAlofPa.]

Great explanation Doug. Do you know if Xtreme and Accura outdoors plated have a soft core? Thanks Al.

[Silver Jack Hammer]

Jacketed boolit discussions usually aren’t welcome around here, but dang, great OP, question and discussion.

[Moleman-]

For what it's worth, I've run into obturation with jacketed pistol bullets in several rimless straight walled cartridges when run at rifle pressures. The "rings" in the pic are from when the 200gr .400" XTP expanding up more than the .002" difference between it and the freebore. The expanded bullet shaving off an "O" as it passed by the sharp case mouth step in the chamber of the 400FUW. Working the loads up the chamber pressure by the time the rings showed up was about 50Kpsi. The freebore on that chamber is .002" over bullet size so it would of had to start expanding up at a lower pressure. Polished the knife edge off of the case mouth step and was about to push that bullet up to about 57Kpsi with no more jacket shaving. The single pointer bullet in the center is a solid copper Barnes that didn't leave a copper ring in the chamber.

[DougGuy]

For what it's worth, I've run into obturation with jacketed pistol bullets in several rimless straight walled cartridges when run at rifle pressures. The "rings" in the pic are from when the 200gr .400" XTP expanding up more than the .002" difference between it and the freebore. The expanded bullet shaving off an "O" as it passed by the sharp case mouth step in the chamber of the 400FUW. Working the loads up the chamber pressure by the time the rings showed up was about 50Kpsi. The freebore on that chamber is .002" over bullet size so it would of had to start expanding up at a lower pressure. Polished the knife edge off of the case mouth step and was about to push that bullet up to about 57Kpsi with no more jacket shaving. The single pointer bullet in the center is a solid copper Barnes that didn't leave a copper ring in the chamber.

The 480 Ruger has a sharp step at the case mouth, which I ream out to a 6 degree 30min angle so it doesn't ring with cast but I'm not aware of any of the factory J-words ringing the chambers with gilding metal, and I promise you there is obturation going on there before the bullet leaves the case mouth. Maybe the jackets are thicker?

[lightload]

Thank you all for try your input.

[Norske]

I'd expect it to happen with FMJ bullets with exposed lead at the base.

[Lloyd Smale]

very little if anything and thats what your goal should be in a cast bullet too. Soft bullets that (i hate this term) bump up are bullets that have deformed and how do expect to shoot a great group with a bullet that is a blob of lead. Bullet designer design great bullets and people cast them out of soft alloys and by the time they make the trip down the bore there design has changed. Its why hard bullets in a GOOD gun about allways outshoot softer alloys. Even at low pressures. Some might argue that but ill say this. Ive proved it over and over and over in the last 50 years. Only time soft outshoots hard is in a gun with miss matched dimensions and those guns go down the road. I shot ppc for years with 45acps and 38s and about allways loaded ww for practice and my match ammo was always loaded with bullets made out of linotype. When practicing i just took into account the slightly different point of impact. Its why casters are always chasing jacketed bullet accuracy. you dont go out and try to find softer jacketed bullets for low pressure loads. If they would shoot better companys would have caught on and offered them to comp shooters. So to answer the original question. Yup they "deform" slightly but not as much as all but the hardest of cast alloys and thats a good thing. Contrary to what some old wives tales tell you about how good a bullet that bumps up is.

[MT Gianni]

I remember McMillian writing about a rifle blow up that happened with someone using 45 gr of BE rather than an IMR stick powder in a 270 cartridge. As expected the barrel, action and gun were toast. The kicker is that the firearm was a 30-06 that the 270 cartridge was fired in. Something obdurated in the the bore to prevent that overpressured situation to not just vent past the bullet.

[littlejack]

IMHO; "To much pressure to fast".
Just my .02.

[Sig556r]

There will always be obturation if bullet expands on ignited propellant, gas cutting (main cause of leading) however, depends if obturation is enough to seal the barrel tight for efficient pressure buildup. That's why some folks argue that boattail or bevelled bullets may not be as efficient in this regard as flatbase or hollowbase. As always, fit is king when matching bullet to barrel.

[Norske]

Richard Lee's reloading manual has an excellent article about peak pressure, bullet harness, and filling the bore. Okay, he's advertising his inexpensive hardness tester, but the article is still very good.

[Lloyd Smale]

a 45 cal bullet sized to 452 is already big enough to fill the bore of a barrel cut right. Its why jacketed bullets are only 451. As does a 358 bullet in a proper 357 barrel or a 356 in a a 355 barrel. If i get a gun that needs special bullets casted for it it goes down the road unless its some family heirloom. Ive got multiple guns in about every caliber and dont need any special needs projects.