The term "Bumping up"

[JWFilips]

I have head this term thrown around a lot over the years but really never could understand how one can squash a bullet down on it's long axis and still maintain it's straightness. I guess perhaps there can be something the opposite of a sizing die to squeeze the alloy under pressure to fill the larger size die but I have also heard it in context of bumping to changing the meplat or a cast bullet being placed between the jaws of a vice and squeezed a quarter tun of the handle. A knowledgeable explanation would be most appreciated.

 

[RicinYakima]

"Bumping up" has a generic definition of reforming a cast bullet to a larger diameter. Benchrest shooters sometimes use a forming die that does look like a sizing die in their lube and sizer. The bullet goes in nose first and as pressure is applied to the base, the bullet expands to he fill the cavity. While I have never heard of using a vise, you can use different nose punches to change the shape of the nose while sizing. All of this works best with softer bullets, as you will break the lube/sizer with hard/heat treated bullets.

 

[Ian]

I have head this term thrown around a lot over the years but really never could understand how one can squash a bullet down on it's long axis and still maintain it's straightness.

Not very well, most of the time. It can be done when firing but the dynamic fit has to be just right.
Nose-bumping, either of the two ways as Ric described, is the other option. Vise? No way.

 

[fiver]

think of it as swaging and not sizing.
for instance I have a mold for my 44-40, it makes the traditional 428 diameter and casts to about 200 grains.
my 44-40 barrel is almost 430 diameter so 428 didn't really work for me unless I was casting soft and slamming the bullet hard with my powder choices.
so what I do now is to run them through my lube sizer, then I put them in my 4305 swage die and press on the base letting the pressure squeeze everything up to size and load them as normal.
sometimes I put a hollow point on them to shoot in the 44 sp. revolvers since this pushes the nose diameter out just a bit more and is a closer match to my cylinder throat shape.

what the bench rest guys are doing is using a similar setup except their swage die is made from the reamer that cut the throat in their rifle [and they cut their size die for their cases with their chamber cutter tool]
they make and size-lube their custom bullet just like the rest of us,,,, but then they pre-form the bullet in the swage die to exactly match their rifles throat. [bumping it up in diameter somewhere]
they seat that bullet so that everything is an exact copy of their rifle only a little shorter in length out front.

 

[KHornet]

Fivers explination is the one I would go with.
There are variations of bumping however,
depending on the whims of the person
doing the bumping.

Paujl

 

[popper]

how one can squash a bullet down on it's long axis and still maintain it's straightness.
Sizing, swaging, shooting are all the same. Lead is incompressable but does cold flow. Sizing, the nose/'mass' of the bullet is alowed to move. Swaging, mass held in place. shooting, mass has no initial inertia (takes energy to get it to move) but the base moves first, like swaging. The nose of pointy cast can get 'off center' when firing and most 'nose' engraving is from bumping. The barrel will mostly keep it straight but the base can get deformed

 

[Intheshop]

On a nose bump,I look at it as starting the inevitable..... as deformation on launch.Goes along with a jam fit on loaded rounds.

Oh yeah,the extreme levels of "squareness" is accomplished with a perfect lower die.