When members of the House and Senate Armed Services committees meet in conference to thrash out conflicts in their respective defense budgets, they may wind up arguing about grocery shopping.

Early last week, the Senate Armed Services Committee passed a provision to expand the commissary shopping privileges of the Selected Reserve. At about the same time, a House subcommittee unanimously rejected a similar provision.

The commissaries are huge markets subsidized by the government that sell household items to military personnel at more than a 20 percent discount. The Senate committee provision would enable reservists to make 14 trips to the market, at any time of the year. Currently, reservists can use the commissary only when they are on active duty. Reservists serve two weeks of active duty each year, plus a weekend a month for drills.

By seeking to grant the reservists 14 trips down the vegetable and meat aisles, sponsors Rep. G.V. (Sonny) Montgomery (D-Miss.) and Sen. J. James Exon (D-Neb.) hope to improve morale and retain members. In a Senate speech, Exon quoted a Rand Corp. survey saying that more than 31 percent of those who quit the reserves cited "family and lesiure time demands" as the primary reasons.

"With the guards and the reserve playing the key role that they do, unprecedented as far as history is concerned, it is vitally important that we keep our guards and reservists happy and in their positions," Exon said in an interview.

Nevertheless, officials in the Office of Management and Budget and the Defense Department found the commissary proposal hard to digest. An OMB report said "there are more efficient and less controversial ways" to improve morale. The Defense Department agreed and also opposed the plan.

One reason for opposition is cost. A Defense Department analysis predicted Congress might have to add as much as $166 million to the current $668 million commissary appropriation if 75 percent of the nation's 1,147,177 reservists took advantage of the privileges.

"Numbers tend to confuse people," Exon responded. "I can't imagine that this is going to be used nearly as much as some people figure, but if it's used in its entirity by every guard and every reservist, I can't imagine it would increase the cost of commissaries at all."

An Exon aide said expanded privileges will pay for themselves if morale improves and the military does not have to spend more resources on recruiting and retraining.

According to one Pentagon official, many in the Defense Department have opposed the provision because they fear that expansion could antagonize civilians and jeopardize the commissary system.

Opposition also is coming from the private sector. The Food Marketing Institute, whose clients sell 50 percent of the groceries in the nation, said that if 75 percent of the reservists use the commissaries, the private sector could lose $1.38 billion in sales annually.

A similar provision died in conference last year, but Andre Clemandot Jr., Rep. Montgomery's administrative aide, said the congressman is pressing again this year. "My boss still believes it is a strong family support item, and that's why he supports it.