But which guns?

Something like a consensus has been reached in Washington over one feature of President Reagan's military program. Almost everyone, including the President himself, agrees that it is too big and must be trimmed.

But the battle over where and what to cut is just being joined.

The original Reagan program avoided hard choices over weapons and weapons systems by providing just about everything anyone wanted, particularly the aerospace industry. In rough terms the President simply ordered whatever the industry thought it could provide.

And that rule still stands. Last week Defense Secretary Caspar Weinberger announced that he had succeeded in trimming back the 1984 fiscal year budget by his trimming. He claimed that he got most of the trim by recalculating inflation and fuel costs. The rest he thought he could get by holding back on military pay increases, by canceling some training exercises, and by slowing some military construction.

But he is not willing, yet, to think in terms of canceling any of the three most controversial items in the program which happen, also, to be the most costly.

Those three items are the darlings of the aerospace industry, much of which is located in southern California, hence of particular interest to any former governor of California and to the leader of the right wing of the Republican Party. Southern California is probably the staunchest bastion of right-wing Republicanism to be found anywhere in the country, hence the home constituency of the President.

So Mr. Weinberger is willing to trim a little from pay increases, desirable for troop morale, and from training, which is vital for combat readiness. But the three great sacred cows come through this first round unscratched.

Are those three really essential?

Most controversial of all is the B-1 bomber since it is only an interim weapon intended to fill the gap between the aging B-52s and the prospective ''Stealth'' bomber. But since ''Stealth'' is coming along supposedly quite well and since B-52s are still the fastest, highest-flying bombers in the world, is it really necessary to build the interim B-1 which will be useless once ''Stealth'' comes along?

Besides, the case for having any manned, long-range bomber declines as the new cruise missiles come into production. An unmanned cruise missile can reach any target in the Soviet Union. It is cheap. It can be launched from land, sea, or air. It is totally mobile. Why then send a manned bomber through the air defenses of some enemy frontier?

The Air Force has arguments for the manned bomber. One is that it can be recalled up to the last moment. But is that worth the cost? The arguments for it are more political than technical.

The MX comes next. It is too big to be moved around. It is a fixed, land-based weapon. The Air Force wants it. The Army, Navy, and Marine Corps representatives on the Joint Chiefs of Staff voted against it. They favor mobile missiles. The MX is big enough to carry 10 warheads, each of 350 kilotons yield. But it is not mobile. A ballistic missile capable of carrying a single warhead of the same 350-kt. yield would be mobile. It could be trundled around on an ordinary truck, by road or rail.

The extra two big carriers are also controversial.

They have high appeal to the aircraft industry since each would carry an entire air wing of 70 to 95 aircraft, with replacements always in the pipeline. They have considerable value for supporting long operations in faraway places, such as the Indian Ocean. But the present fleet is built around 12 carriers. Is there a real need for going up to a 14-carrier level, requiring an appropriate increase in escort destroyers?

The Navy is divided over more big carriers. Some regard them as still being the backbone of the fleet. Others feel that a greater need is for many, small ships designed to protect the sea lanes against enemy submarines. Those who favor sea lane control ships regard an extra two big carriers as pure luxury. No other country has 12 big aircraft carriers.

Many experts think that all three of the super weapons could be scrapped without serious loss to anyone but the aerospace industry. Cutting out all three would not balance the budget, but it would help.